THE HAGUE (Reuters) -- Islamic State militants "most probably" used the banned chemical weapon mustard gas against Kurdish forces in Iraq and international inspectors have been asked to investigate, diplomatic sources told Reuters.
A team of inspectors from the Organisation for the Prohibition of Chemical Weapons (OPCW) will go to Iraq next week to help determine if the blistering agent sulfur mustard was used in the battlefield, three sources told Reuters.
Islamic State already has a broad arsenal of weapons and military vehicles seized from Iraqi forces. It launched an attack with "weaponized chlorine" in January and there have been at least four other occasions it allegedly used chemical weapons, regional officials said.
If Islamic State insurgents have obtained sulfur mustard, commonly known as mustard gas, it would signal a dangerous development in a conflict that has already destabilized the region.
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Exclusive: Watchdog to probe alleged mustard gas use by Islamic state
RNPS: YEAREND REVIEW 2014 - HEADLINE MAKERS Militant Islamist fighters hold the flag of Islamic State (IS) while taking part in a military parade along the streets of northern Raqqa province in this June 30, 2014 file photo. 2014 saw the rise of the Sunni militant group Islamic State, which has seized swathes of territory in both Syria and Iraq.
A man purported to be Islamic State captive Jordanian pilot Muath al-Kasaesbeh (in orange jumpsuit) stands in front of armed men in this still image from an undated video filmed from an undisclosed location made available on social media on February 3, 2015. Islamic State militants released the video on Tuesday purporting to show Kasaesbeh being burnt alive, and Jordanian state television said he was murdered a month ago. Reuters could not immediately confirm the video, which showed a man resembling the captive pilot standing in a black cage before being set ablaze.
(REUTERS/Social media via Reuters TV)
Smoke rises over Syrian town of Kobani after an airstrike, as seen from the Mursitpinar border crossing on the Turkish-Syrian border in the southeastern town of Suruc in Sanliurfa province, October 18, 2014. A U.S.-led military coalition has been bombing Islamic State fighters who hold a large swathe of territory in both Iraq and Syria, two countries involved in complex multi-sided civil wars in which nearly every country in the Middle East has a stake.
This handout image provided by the Iraqi Prime Minister office shows Iraq's Prime Minister Nouri al-Maliki holding photographs of a man identified by the Iraqi government as al-Qaida leader in Iraq Abu Omar al-Baghdadi at a news conference on April 19, 2010 in Baghdad, Iraq. Nouri announced the deaths of Abu Ayyub al-Masri along with Abu Omar al-Baghdadi. US Military oficials confirmed U.S. and Iraqi forces killed the two al-Qaida figures in a nighttime rocket attack on a safe house near Tikrit.
(Photo by Iraqi Prime Minister office via Getty Images)
BAGHDAD, IRAQ, FEBRUARY 21: An Iraqi security officer patrols the grounds at the newly opened Baghdad Central Prison in Abu Ghraib on February 21, 2009 in Baghdad, Iraq. The Iraqi Ministry of Justice has renovated and reopened the previously named 'Abu Ghraib' prison and renamed the site to Baghdad Central Prison. According to the Iraqi Ministry of Justice about 400 prisoners were transferred to the prison which can hold up to 3000 inmates. The prison was established in 1970 and it became synonymous with abuse under the U.S. occupation.
(Photo by Wathiq Khuzaie/Getty Images)
An Iraqi soldier guards the site where allegedly top Al-Qaeda leaders in Iraq, Abu Omar al-Baghdadi and Abu Ayyub al-Masri, were killed in a joint Iraqi-US military raid in Al-Dhahiriya in Salaheddin province, 280 kms north of Baghdad, on April 20, 2010. Iraqi Prime Minister Nuri al-Maliki and the US military said on April 19 that Baghdadi and Masri were killed in a raid on a safehouse, which yielded computers filled with emails and messages to bin Laden and his deputy Ayman al-Zawahiri. AFP PHOTO/MAHMUD SALEH (Photo credit should read Mahmud Saleh/AFP/Getty Images)
A shrine for Iraqi Christians who were killed in Al-Qaeda siege is erected at the Sayidat al-Nejat (Our Lady of Salvation) Church in Baghdad on December 23, 2010 as Christmas for Iraq's Christian community will this year be a time of fear and cancelled celebrations instead of rejoicing following renewed threats by Al-Qaeda and the church massacre.
(ALI AL-SAADI/AFP/Getty Images)
An Iraqi woman walks past destroyed shops on the ground floor of a building the day after twin car bombs in the Karrada area of the capital Baghdad on August 1, 2012, in which some 12 people were killed. July was the deadliest month in Iraq in almost two years, with 325 people killed in attacks, and included the deadliest day here since December 2009, official figures released showed.
A lorry drives past a sign welcoming people to the 'Islamic State of Gao' at the entrance of the northern Malian city of Gao, on March 9, 2013. After nine months of occupation by Islamic militants, and subsequent liberation by French troops, Gao is slowly shedding its hardline Islamic way of life, with bars serving beer making a long awaited comeback. AFP PHOTO / JOHN MACDOUGALL (Photo credit should read JOHN MACDOUGALL/AFP/Getty Images)
A fighter of the Kurdish People's Protection Units (YPG) fires an anti-aircraft weapon from Tel Tawil village in the direction of Islamic State fighters positioned in the countryside of the town of Tel Tamr February 25, 2015. Kurdish militia pressed an offensive against Islamic State in northeast Syria on Wednesday, cutting one of its supply lines from Iraq, as fears mounted for dozens of Christians abducted by the hardline group. The Assyrian Christians were taken from villages near the town of Tel Tamr, some 20 km (12 miles) to the northwest of the city of Hasaka. There has been no word on their fate. There have been conflicting reports on where the Christians had been taken.
German alleged jihadist Kreshnik B (R) listens to his lawyer Mutlu Guenal (L) as he arrives at the higher regional court in Frankfurt am Main, western Germany, on September 15, 2014 on the opening of his trial on charges of fighting for Islamic State (IS) in Syria, in Germany's first court proceedings involving the militant group. The defendant was arrested in December 2013 at Frankfurt airport in western Germany on his way home from Syria.
(THOMAS KIENZLE/AFP/Getty Images)
An image grab taken from an AFPTV video on September 16, 2014 shows a jihadist from the Islamic State (IS) group standing on the rubble of houses after a Syrian warplane was reportedly shot down by IS militants over the Syrian town of Raqa. The plane crashed into a house in the Euphrates Valley city, the sole provincial capital entirely out of Syrian government control, causing deaths and injuries on the ground.
Smoke rises in the distance behind an Islamic State (IS) group flag and banner after Iraqi Kurdish Peshmerga fighters reportedly captured several villages from IS group jihadists in the district of Daquq, south of the northern Iraqi multi-ethnic city of Kirkuk on September 11, 2015. An Iraqi officer said that the operation was launched in the morning with support from international coalition aircraft, and has succeeded in retaking ten villages from IS.
(MARWAN IBRAHIM/AFP/Getty Images)
Iraqi Shiite fighters from the Hashed al-Shaabi (Popular Mobilisation) paramilitaries drive a T-72 tank as they advance near the town of Tal Abtah, south of Tal Afar, on November 30, 2016 during a broad offensive by Iraq forces to retake the city Mosul from jihadists of the Islamic State group.
(AHMAD AL-RUBAYE/AFP/Getty Images)
Lt. Gen. William C. Mayville Jr., Joint Staff Director of Operations Director of Operations, shows before and after photos as he speaks on the airstrikes in Syria during a briefing at the Pentagon September 23, 2014 in Washington, DC. Mayville briefed the press about US and Arab nation joint airstrikes against Islamic State(IS) group targets in Syria and unilateral airstrikes against an al-Qaeda group in Syria.
(BRENDAN SMIALOWSKI/AFP/Getty Images)
Men in orange jumpsuits purported to be Egyptian Christians held captive by the Islamic State (IS) kneel in front of armed men along a beach said to be near Tripoli, in this still image from an undated video made available on social media on February 15, 2015. Islamic State released the video on Sunday purporting to show the beheading of 21 Egyptian Christians kidnapped in Libya. In the video, militants in black marched the captives to a beach that the group said was near Tripoli. They were forced down onto their knees, then beheaded. Egypt's state news agency MENA quoted the spokesman for the Coptic Church as confirming that 21 Egyptian Christians believed to be held by Islamic State were dead.
(REUTERS/Social media via Reuters TV)
A fighter of the Islamic State of Iraq and the Levant (ISIL) holds an ISIL flag and a weapon on a street in the city of Mosul, June 23, 2014. U.S. Secretary of State John Kerry held crisis talks with leaders of Iraq's autonomous Kurdish region on Tuesday urging them to stand with Baghdad in the face of a Sunni insurgent onslaught that threatens to dismember the country. Picture taken June 23, 2014.
Members of the Saraya al-Salam (Peace Brigades), a group formed by Iraqi Shiite Muslim cleric Moqtada al-Sadr, march in Iraq's holy city of Najaf as they prepare to reinforce government forces in the fight against the Islamic State group for control of Fallujah, east of the capital, on May 17, 2016. Iraqi security forces and allied fighters have regained significant ground from the jihadists, securing the Ramadi area earlier this year and retaking the town of Heet last month. But parts of Anbar -- including Fallujah -- are still under IS control, as is most of Nineveh province, to its north.
(HAIDAR HAMDANI/AFP/Getty Images)
An Iraqi Policeman watches a drone hover near the village of Arbid, on the southern Mosul front, on November 12, 2016 during the ongoing military operation to retake Mosul from the Islamic State (IS) group. Iraqi forces launched a massive operation to retake the country's second city from the Islamic State group on October 17, and the Counter-Terrorism Service (CTS) special forces have pushed the jihadists back from some Mosul neighbourhoods.
(ACHILLEAS ZAVALLIS/AFP/Getty Images)
A MI 28 provides aerial support as Shiite fighters from the Hashed al-Shaabi (Popular Mobilisation), enter the village of al-Tofaha, southeast of the city of Tal Afar, on November 25, 2016, during an ongoing operation against Islamic State (IS) group jihadists. The Popular Mobilisation have focused their operations on Tal Afar, a large town still held by IS west of Mosul and this week announced they had cut the main road between it and Syria.
(AHMAD AL-RUBAYE/AFP/Getty Images)
Shiite fighters stands near a wall painted with the black flag commonly used by Islamic State militants in the town of Tal Ksaiba, near the town of al-Alam, March 7, 2015. Iraqi security forces and Shi'ite militia fighters struggled to advance on Saturday into the two towns of al-Alam and al-Dour near Tikrit, their progress slowed by fierce defence from Islamic State militants.
F16 fighter jets from the United Arab Emirates (UAE) arrive at an air base in Jordan February 8, 2015. A squadron of F16 jet fighters from the United Arab Emirates arrived in Jordan on Sunday a day after the Gulf state announced it was being sent to bolster the coalition's military effort against the Islamic State. It will conduct joint air strikes with Jordanian colleagues against the Islamic militants, Jordanian officials said on Saturday.
(REUTERS/Petra News Agency)
Lt. Gen. William C. Mayville Jr. speaks about the Syrian bombing campaign September 23, 2014 in Washington, DC. Mayville talked about the U.S. and Arab air strikes in Syria against the jihadist group Islamic State in Iraq and Syria (ISIS).
(Photo by Mark Wilson/Getty Images)
A girl holds up a poster with pictures of the 21 Egyptian Coptic Christians beheaded by Islamic State in Libya, as they gather in a gesture to show their solidarity, in front of the Egyptian embassy in Amman February 17, 2015.
(REUTERS/ Muhammad Hamed)
Shi'ite fighters, from the brigades of peace loyal to Shi'ite cleric Moqtada al-Sadr, who have joined the Iraqi army to fight against militants of the Islamic State, formerly known as the Islamic State of Iraq and the Levant (ISIL), take part in field training in Najaf, August 23, 2014.
A Kurdish fighter keeps guard while overlooking positions of Islamic State militants near Mosul in northern Iraq August 19, 2014. Sunni Muslim fighters led by the Islamic State swept through much of northern and western Iraq in June, capturing the Sunni cities of Tikrit and Mosul as well as the Mosul dam, which controls water and power supplies to millions of people down the Tigris river valley.
AL-QARYATAYN, SYRIA. APRIL 7, 2016. The Mar Elian Catholic monastery burnt by Islamic State (IS) militants.
(Photo by Valery Sharifulin\TASS via Getty Images)
Shi'ite fighters look at smoke rising from clashes during a battle with Islamic State militants at the airport of Tal Afar west of Mosul, Iraq November 18, 2016.
Indonesian soldiers from the 2nd Airborne Division patrol after parachuting from a transport aircraft near Masani village, Poso, Central Sulawesi March 31, 2015 in this photo taken by Antara Foto. Indonesia has launched military exercises in the eastern island of Sulawesi, a haven for radical Islamists, as part of broader efforts to crack down on militants with suspected links to the Islamic State group, officials said on Tuesday. The drills come amid heightened government concerns over a rising number of Indonesians pledging loyalty to Islamic State (IS) and trying to join the group fighting in Iraq and Syria.
An Iraqi soldier holds an Islamist State flag, after pulling it down during a military operation against Islamic State militants in Al-Qasar, southeast of Mosul, Iraq, November 29, 2016.
A man walks in a street with abandoned vehicles and damaged buildings in the northern Syrian town of Kobani January 30, 2015. Sheets meant to hide residents from snipers' sights still hang over streets in the Syrian border town of Kobani, and its shattered buildings and cratered roads suggest those who fled are unlikely to return soon. Kurdish forces said this week they had taken full control of Kobani, a mainly Kurdish town near the Turkish border, after months of bombardment by Islamic State, an al Qaeda offshoot that has spread across Syria and Iraq.
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Sulfur mustard is a Class 1 chemical agent, which means it has few uses outside chemical warfare. A blistering agent that causes severe delayed burns to the eyes, skin and respiratory tract, it was used on a massive scale during World War One.
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Islamic State militants "most probably" used the banned chemical weapon mustard gas against Kurdish forces in Iraq
Kurdish authorities say 35 troops southwest of the regional capital of Erbil were sickened in August by a chemical attack. Blood samples tested positive for mustard gas, they said earlier this month.
"Most probably it was mustard gas," said one diplomat, who spoke on condition of anonymity because the information was classified, "But we want to make sure and we want to know where it came from because it is very difficult to get."
Chemical agents are not believed to have been used in fighting in Iraq since Saddam Hussein's fall in 2003.
If confirmed by the OPCW, it would raise serious questions about how Islamic State militants got their hands on the chemicals and, crucially, where they came from.
There are no declared stockpiles of mustard gas in the Middle East since Syria joined the Chemical Weapons Convention in 2013, vowing to eliminate its chemical weapons program.
The OPCW team's mandate will be limited to the single allegation of use in August against the Kurdish Peshmerga forces, which was allegedly fired using mortar rounds.
Syria declared 1,300 tonnes of chemicals weapons to the OPCW in 2013 as part of a deal with Russia, avoiding air strikes that had been threatened by the United States.
Amid continued allegations of the use of prohibited chlorine barrel bombs in Syria, Damascus is in discussions with the OPCW about whether it accurately and fully disclosed its chemical weapons program.