US: Syria may be developing new types of chemical weapons

WASHINGTON (Reuters) - The Syrian government may be developing new types of chemical weapons, and U.S. President Donald Trump is prepared to again order military action against Syrian leader Bashar al-Assad's forces if necessary to deter chemical attacks, senior U.S. officials said on Thursday.

Assad's government is believed to have kept part of its chemical weapons stockpile despite a U.S.-Russian deal under which Damascus was supposed to have handed over all those weapons for destruction in 2014, the officials said.

Assad's forces have continued occasional use of chemical weapons in smaller amounts since a deadly attack last April that drew a U.S. missile strike on a Syrian air base, the officials told reporters in a briefing.

RELATED: A look back at the deadly Syria gas attack in April of 2017

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Syria gas attack
ATTENTION EDITORS - VISUAL COVERAGE OF SCENES OF INJURY OR DEATH A man carries the body of a dead child, after what rescue workers described as a suspected gas attack in the town of Khan Sheikhoun in rebel-held Idlib, Syria April 4, 2017. REUTERS/Ammar Abdullah
ATTENTION EDITORS - VISUAL COVERAGE OF SCENES OF INJURY OR DEATH People stand near a dead body, after what rescue workers described as a suspected gas attack in the town of Khan Sheikhoun in rebel-held Idlib, Syria April 4, 2017. REUTERS/Ammar Abdullah
A man breathes through an oxygen mask as another one receives treatments, after what rescue workers described as a suspected gas attack in the town of Khan Sheikhoun in rebel-held Idlib, Syria April 4, 2017. REUTERS/Ammar Abdullah
A man breathes through an oxygen mask, after what rescue workers described as a suspected gas attack in the town of Khan Sheikhoun in rebel-held Idlib, Syria April 4, 2017. REUTERS/Ammar Abdullah
ATTENTION EDITORS - VISUAL COVERAGE OF SCENES OF INJURY OR DEATH Men stand near dead bodies, after what rescue workers described as a suspected gas attack in the town of Khan Sheikhoun in rebel-held Idlib, Syria April 4, 2017. REUTERS/Ammar Abdullah
EDITORS NOTE: Graphic content / A Syrian child receives treatment at a hospital in Khan Sheikhun, a rebel-held town in the northwestern Syrian Idlib province, following an attack on April 4, 2017. A suspected chemical attack killed at least 58 civilians including several children in rebel-held northwestern Syria, a monitor said, with the opposition accusing the government and demanding a UN investigation. / AFP PHOTO / Omar haj kadour (Photo credit should read OMAR HAJ KADOUR/AFP/Getty Images)
A man holds an injured baby inside a Turkish ambulance as injured Syrian people enter into Turkey from the Cilvegozu border gate in Hatay province, near the Syrian border on April 4, 2017. Turkish President Recep Tayyip Erdogan in a phone call with Russian counterpart Vladimir Putin on April 4, 2017 condemned a suspected chemical attack in northwestern Syria as an 'inhuman' strike that could endanger peace talks based in the Kazakh capital. / AFP PHOTO / DOGAN NEWS AGENCY / - / Turkey OUT (Photo credit should read -/AFP/Getty Images)
IDLIB, SYRIA - APRIL 4: A wounded kid receives medical treatment at sahra hospital after Assad Regime forces's attack with chlorine gas to Khan Shaykhun town of Idlib, Syria on April 4, 2017. (Photo by Abdulghani Arian/Anadolu Agency/Getty Images)
IDLIB, SYRIA - APRIL 4: A woman gets treatment at a hospital after Assad Regime forces attacked with suspected chlorine gas to Khan Shaykhun town of Idlib, Syria on April 4, 2017. (Photo by Bahjat Najar/Anadolu Agency/Getty Images)
IDLIB, SYRIA - APRIL 4: A child gets treatment at a hospital after Assad Regime forces attacked with suspected chlorine gas to Khan Shaykhun town of Idlib, Syria on April 4, 2017. (Photo by Bahjat Najar/Anadolu Agency/Getty Images)
ATTENTION EDITORS - VISUAL COVERAGE OF SCENES OF INJURY OR DEATH A man carries the body of a dead child, after what rescue workers described as a suspected gas attack in the town of Khan Sheikhoun in rebel-held Idlib, Syria April 4, 2017. REUTERS/Ammar Abdullah TPX IMAGES OF THE DAY
A crater is seen at the site of an airstrike, after what rescue workers described as a suspected gas attack in the town of Khan Sheikhoun in rebel-held Idlib, Syria April 4, 2017. REUTERS/Ammar Abdullah TPX IMAGES OF THE DAY
Turkish officials with chemical clothes carry a injured man on April 4, 2017 in Hatay province, near the Syrian border. Turkish President Recep Tayyip Erdogan in a phone call with Russian counterpart Vladimir Putin on April 4, 2017 condemned a suspected chemical attack in northwestern Syria as an 'inhuman' strike that could endanger peace talks based in the Kazakh capital. / AFP PHOTO / DOGAN NEWS AGENCY / - / Turkey OUT (Photo credit should read -/AFP/Getty Images)
EDITORS NOTE: Graphic content / Bodies lie in the parking area of a hospital in Khan Sheikhun, a rebel-held town in the northwestern Syrian Idlib province, following a suspected toxic gas attack on April 4, 2017. A suspected chemical attack killed at least 58 civilians including several children in rebel-held northwestern Syria, a monitor said, with the opposition accusing the government and demanding a UN investigation. / AFP PHOTO / Omar haj kadour (Photo credit should read OMAR HAJ KADOUR/AFP/Getty Images)
IDLIB, SYRIA - APRIL 4: A child gets treatment at a hospital after Assad Regime forces attacked with suspected chlorine gas to Khan Shaykhun town of Idlib, Syria on April 4, 2017. (Photo by Bahjat Najar/Anadolu Agency/Getty Images)
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Characteristics of some of those recent attacks suggest that Syria may be developing new weapons and methods for delivering poison chemicals, possibly to make it harder to trace their origin, the officials said, speaking on condition of anonymity. But they declined to provide specifics.

A deadly sarin attack on a rebel-held area in April prompted Trump to order a missile strike last year on the Shayrat air base, from which the Syrian operation is said to have been launched.

"We reserve the right to use military force to prevent or deter the use of chemical weapons," one official said, while declining to specify how serious a chemical attack would have to be to draw a fresh U.S. military response.

If the international community does not act quickly to step up pressure on Assad, Syria's chemical weapons could spread beyond Syria and possibly even "to U.S. shores," a second official said.

SEE ALSO: US to resume refugee admissions from 11 'high-risk' countries

"It will spread if we don't do something," the official warned.

The officials reiterated U.S. Secretary of State Rex Tillerson's recent accusation that Russia, Assad's ally in Syria's multi-sided civil war, bears some responsibility for failing to enforce the chemical weapons ban.

Russia has denied any complicity, and the Syrian government has said it has not carried out any of the attacks.

Most recently, Western officials have cast suspicion on the Syrian government for a chlorine gas attack on a rebel-held enclave east of Damascus last week that sickened at least 13 people.

The U.S. officials also said Islamic State militants have used chemical weapons such as sulfur mustard and chlorine and have employed improvised explosive devices to deliver the chemicals.

RELATED: Heartbreaking scenes from Syria

17 PHOTOS
Heartbreaking scenes from Eastern Ghouta, Syria
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Heartbreaking scenes from Eastern Ghouta, Syria
Six-month-old twins Safa and Marwa, who suffer from malnutrition, are seen at their home in the Hazzeh area, in the eastern Damascus suburb of Ghouta, Syria, October 25, 2017. REUTERS/Bassam Khabieh SEARCH "KHABIEH MALNUTRITION" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES.
Two-and-a-half year old Hala al-Nufi, who suffers from a metabolic disorder which is worsening due to the siege and food shortages in the eastern Ghouta, reacts as she sits on a bed in the Saqba area, in the eastern Damascus suburb of Ghouta, Syria, October 25, 2017. REUTERS/Bassam Khabieh SEARCH "KHABIEH MALNUTRITION" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. TPX IMAGES OF THE DAY.
A Syrian shovels away debris from the higher floor of a building that was reportedly shelled by regime forces in the rebel-held Eastern Ghouta town of Kafr Batna on the outskirts of Damascus on November 20, 2017. / AFP PHOTO / ABDULMONAM EASSA (Photo credit should read ABDULMONAM EASSA/AFP/Getty Images)
ATTENTION EDITORS - VISUALS COVERAGE OF SCENES OF DEATH AND INJURY An injured student lies on a bed at a field hospital in the town of Jisreen, in the eastern Damascus suburb of Ghouta, Syria October 31, 2017. REUTERS/Bassam Khabieh TEMPLATE OUT
DAMASCUS, SYRIA - NOVEMBER 20: People inspect the debris of a building after Assad regime's warcrafts carried out airstrikes over residential areas of Kafr Batna town of the Eastern Ghouta region of Damascus in Syria on November 20, 2017. It is reported that 9 civilians, including 4 children were killed after the attack. (Photo by Anas Damashqy/Anadolu Agency/Getty Images)
ATTENTION EDITORS - VISUAL COVERAGE OF SCENES OF INJURY OR DEATH A wounded man is seen lying in Douma hospital after heavy shelling in the rebel-held besieged town of Douma, eastern Ghouta in Damascus, Syria, November 19, 2017. REUTERS/Bassam Khabieh TEMPLATE OUT
ATTENTION EDITORS - VISUAL COVERAGE OF SCENES OF INJURY OR DEATH A wounded man is seen lying in Douma hospital after heavy shelling in the rebel-held besieged town of Douma, eastern Ghouta in Damascus, Syria, November 19, 2017. REUTERS/Bassam Khabieh
A man walks on rubble after an airstrike in the rebel-held city of Douma, in the eastern Damascus suburb of Ghouta Syria November 2, 2017. REUTERS/Bassam Khabieh
Two-and-a-half year old Hala al-Nufi, who suffers from a metabolic disorder which is worsening due to the siege and food shortages in the eastern Ghouta, is held by her uncle in the Saqba area, in the eastern Damascus suburb of Ghouta, Syria, October 25, 2017. REUTERS/Bassam Khabieh SEARCH "KHABIEH MALNUTRITION" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. TPX IMAGES OF THE DAY.
Children wait to be examined from Unicef health workers in Kafra Batna in the Damascus suburb of eastern Ghouta, Syria October 30, 2017. REUTERS/Bassam Khabieh
ATTENTION EDITORS - VISUALS COVERAGE OF SCENES OF DEATH AND INJURY An injured student lies on a bed at a field hospital in the town of Jisreen, in the eastern Damascus suburb of Ghouta, Syria October 31, 2017. REUTERS/Bassam Khabieh TEMPLATE OUT
ATTENTION EDITORS - VISUAL COVERAGE OF SCENES OF INJURY OR DEATH Injured students lie on beds at a field hospital in the town of Jisreen, in the eastern Damascus suburb of Ghouta, Syria October 31, 2017. REUTERS/Bassam Khabieh
Smoke rises at a damaged site in Ain Tarma, eastern Damascus suburb of Ghouta, Syria, September 14, 2017. REUTERS/Bassam Khabieh
Children hold corncobs at Ain Tarma, eastern Damascus suburb of Ghouta, Syria July 19, 2017. REUTERS/Bassam Khabieh
A Syrian child is seen walking near International Red Cross vehicle in the rebel-held city of Douma, in the eastern Damascus suburb of Ghouta, Syria November 12, 2017. REUTERS/Bassam Khabieh
A Syrian woman walks past damaged buildings in Douma, in the eastern Damascus suburb of Ghouta, Syria November 15, 2017. REUTERS/Bassam Khabieh
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(Reporting by Matt Spetalnick; Editing by James Dalgleish)

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