DUBAI/WASHINGTON (Reuters) -- Iran released 10 U.S. sailors on Wednesday after holding them overnight, bringing a swift end to an incident that had rattled nerves days ahead of the expected implementation of a landmark nuclear accord between Tehran and world powers.
Iran's Islamic Revolutionary Guard Corps (IRGC) said it had freed the sailors after determining they had entered Iranian territorial waters by mistake. The sailors had been detained aboard two U.S. Navy patrol boats in the Gulf on Tuesday.
"Our technical investigations showed the two U.S. Navy boats entered Iranian territorial waters inadvertently," the IRGC said in a statement carried by state television. "They were released in international waters after they apologized," it added.
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Iran releases detained US sailors
Iran frees US sailors swiftly as diplomacy smoothes waters
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RGC Rear Admiral Ali Fadavi had earlier said that the two U.S. Navy boats entered Iranian territorial waters due to a broken navigation system.
U.S. Vice President Joe Biden rejected reports that Washington had offered Iran an apology over the incident.
"No, there was no apology, nothing to apologize for ... and there's no looking for any apology," Biden said on CBS's 'This Morning' program.
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Confirming the sailors' safe release, the Pentagon said there were no indications they were harmed while in Iranian custody.
A carefully worded statement did not explain how the sailors and their two riverine command boats ended up being detained by Iran, saying only that "the Navy will investigate the circumstances that led to the sailors' presence in Iran."
The sailors were later taken ashore by U.S. Navy aircraft, while other sailors took charge of the boats and headed towards Bahrain, their original destination.
U.S. Defense Secretary Ash Carter said he was pleased the sailors had been freed and appreciated "the timely way in which this situation was resolved."
"I want to personally thank Secretary of State John Kerry for his diplomatic engagement with Iran to secure our sailors' swift return."
Ash Carter, U.S. Defense Secretary
Kerry thanked Iran for its cooperation in the release of the sailors.
"I think we can all imagine how a similar situation might have played out three or four years ago, and the fact that today this kind of issue can be resolved peacefully and efficiently is a testament to the critical role diplomacy plays in keeping our country safe, secure, and strong," Kerry said.
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Kerry spoke to Iranian Foreign Minister Mohammad Javad Zarif several times as the United States sought to win the release of the sailors, a U.S. official said.
Zarif said on twitter that he was "happy to see dialog and respect, not threats and impetuousness, swiftly resolved the sailors episode."
Iranian state television released footage of the arrest, showing the sailors as they knelt down with hands behind their heads and their two vessels being surrounded by several IRGC fast boats.
The video showed weapons and ammunition confiscated from the sailors, who were seen eating food provided by the Iranians. There were also images of American passports being inspected.
The incident raised tensions between Iran and the United States, which, along with other world powers, reached a deal last year under which Iran will curb its nuclear activities in exchange for the lifting of economic sanctions.
Some conservatives in both countries, enemies since Iran's 1979 Islamic revolution, have criticized the deal that is due to be implemented in the coming days.
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United States and Iran Relations throughout time
Iran frees US sailors swiftly as diplomacy smoothes waters
Return of the hostages from Iran as they step off the plane in Germany. Barry Rosen is second from the bottom waving with no glasses and a beard. He has just announced that he will be going back to Iran to meet with his captors. The hostages were kept over a year in captivity after the US Embassy was stormed during the Iranian Revolution. Relations with Iran and the US have been improving lately. (photo by Tim Chapman)
A Kurdish family having fled northern Iraq, carry all their worldly possessions, reach the border town of Nossod, Iran. Saddam Hussein crushed the Kurdish uprising in northern Iraq in the aftermath of 1991 Gulf War led by the US and allied forces. (Photo by Kaveh Kazemi/Getty Images)
An Iranian woman pets a dove caged in the belly of a replica of the Statue of Liberty at the former US embassy compound November 3, 2001 in Tehran, Iran. Iranian authorities opened the former embassy to the public for the first time since the 1979 Islamic revolution toppled the US-backed Shah which ultimately led to the breaking of relations between Washington and Tehran. (Photo by Keivan/Getty Images)
Reza Pahlavi, son of the late Shah of Iran Mohammad Reza Shah Pahlavi, speaks to New York University students December 4, 2001 in New York City. Pahlavi spoke of the need to root out terrorism and the need for democracy in Iran. (Photo by Mario Tama/Getty Images)
An Iranian man teaches his son the 'death to America' chant during a demonstration after Friday prayers April 5, 2002 in Tehran. The protesters opposed the Israeli incursion into Palestinian-controlled territories. (Photo by Keivan/Getty Images)
Deputy Secretary of State Richard Armitage greets people before giving testimony before the U.S. Senate Foreign Relations Committee on Capitol Hill October 28, 2003 in Washington DC. The Committee is hearing testimony on security threats and the U.S. policy toward Iran. (Photo by Mark Wilson/Getty Images)
An Iranian girl rests during a meeting of the conservative group, the Coalition of Builders of Islamic Iran, in Tehran February 16, 2004. The group has taken a tough stance on the country's nuclear prerogatives and a moderate line on US-Iranian relations and the imposition of Islamic social regulations. The main reformist parties are staying away after a conservative-run political vetting body, the Guardians Council, barred some 2,300 people -- most of them reformists -- from even standing in the February 20 polls. (Photo credit: BEHROUZ MEHRI/AFP/Getty Images)
A man holds an effigy of U.S. President George W. Bush during a protest against the U.S.-led occupation of Iraq in Enqelab Square May 19, 2004 in Tehran, Iran. Demonstrators reportedly hurled petrol bombs, firecrackers and stones at the British embassy. (Photo by Majid/Getty Images)
This 18 January file photo shows Secretary of State-designate Condoleezza Rice testifying before the Senate Foreign Relations Committee during her confirmation hearing on Capitol Hill in Washington, DC. US Secretary of State Condoleezza Rice will discuss the war in Iraq and tensions with Iran when she meets Chancellor Gerhard Schroeder on 04 February 2005.(Photo credit: TIM SLOAN/AFP/Getty Images)
U.S. Senator Dick Lugar (R-IN) (C) speaks to the media after addressing the U.N. Security Council at the United Nations February 6, 2006 in New York City. Lugar, along with other Senate Foreign Relations Committee members George Voinovich (R-OH) (R) and Norm Coleman (R-MN), spoke on reform at the U.N., Iran and energy conservation. (Photo by Spencer Platt/Getty Images)
Iranian Foreign Minister Manouchehr Mottaki speaks during a meeting with relatives of the seven Iranian detainees arrested in Iraq by US forces, in Tehran, 18 May 2007. US troops seized seven Iranians in the northern Iraqi city of Arbil on 11 January from what Iran claims was an official consular building. Mottaki said after meeting the families of the detainees that Iran maintains that they were diplomats working for a 'consulate'. (Photo credit: ATTA KENARE/AFP/Getty Images)
Iranian students walk past an anti-US mural on the wall of the former US embassy in Tehran 24 October 2007. Twenty-eight years ago, 19-year-old Iranian chemistry student Massoumeh Ebtekar agreed to join other students in holding more than 60 Americans captive at their embassy in Tehran, an event that was to last 444 days and leave a rupture in US-Iranian relations that has yet to be healed. For Ebtekar, who was elected last year as a member of Tehran city council, there is no contradiction between her prominent role in the embassy siege and her efforts today for greater moderation in the Islamic republic. (Photo credit: ATTA KENARE/AFP/Getty Images)
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WASHINGTON, DC - July 09: Undersecretary of State William Burns testifies during the Senate Foreign Relations hearing on Iran. On the same day Iran test-fired missiles in a show of force in the Persian Gulf, the Bush administration Wednesday sought to downplay the country???s power and influence. ???For its part, Tehran seems to relish heightening concerns by promoting the illusion that Iran is on the ascendance,??? Undersecretary of State William Burns said in prepared testimony. ???However, Iran is not 10 feet tall, nor is it even the dominant regional actor.' (Photo by Scott J. Ferrell/Congressional Quarterly/Getty Images)
Japanese Akiko Saberi, mother of US-Iranian journalist Roxana Saberi, holds press cards and ID's for her jailed daughter at the family's house in Tehran on April 18, 2009. An Iranian revolutionary court has sentenced Roxana Saberi, 31, to eight years in jail on charges of spying for the United States, her lawyer said. AFP PHOTO/ATTA KENARE (Photo credit should read ATTA KENARE/AFP/Getty Images)
Iranian nuclear scientist Shahram Amiri stands with his father upon arrival at Imam Khomeini Airport in Tehran on July 15, 2010. Amiri, who claimed he was 'abducted' by US spies last year, denied that he was a nuclear scientist, but said he was questioned by Israelis during his 'harsh' captivity. Amiri had gone missing from Saudi Arabia in June 2009 while on a pilgrimage and surfaced in Iran's Interests Section in Washington on July 13, 2010. Upon his arrival in Tehran he immediately told reporters that he was just a 'simple researcher'. Iranian officials claim he was kidnapped by the Central Intelligence Agency of the United States. US officials have denied these accusations. AFP PHOTO / ATTA KENARE (Photo credit should read ATTA KENARE/AFP/Getty Images)
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Senator John McCain, a Republican from Arizona, right, and U.S. Secretary of State Hillary Clinton talk during a Senate Foreign Relations Committee nomination hearing in Washington, D.C., U.S., on Thursday, Jan. 24, 2013. Senator John Kerry stressed the need to prevent Iran from acquiring nuclear weapons. He described the 'immediate, dangerous challenges' facing the nation as he seeks confirmation to become secretary of state. Photographer: Andrew Harrer/Bloomberg via Getty Images
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Iran's armed forces chief, Major General Hassan Firouzabadi, said the incident should demonstrate Iranian strength to "troublemakers" in the U.S. Congress, which has sought to put pressure on Iran after the nuclear deal.
And at a presidential campaign rally in the United States, Republican front runner Donald Trump, who accuses President Barack Obama of being weak on foreign policy, described the detention of the salors as "an indication of where the hell we're going."
A senior U.S. defense official said the circumstances surrounding the incident were still not entirely clear.
"We haven't been able to fully debrief the sailors," the official said, adding the U.S. military hoped to do so within hours. The sailors were headed to a U.S. military facility in Qatar.
"They're going through what always happens in these cases, they'll get a medical evaluation, and there will be a debriefing."
Attributing the boats' incursion into Iranian waters to a navigation error marked a de-escalation in rhetoric. Earlier, the Guards had said the boats were "snooping" in Iranian territory and Zarif had demanded an apology from Washington.
The IRGC, the Islamic Republic's praetorian guard, is highly suspicious of U.S. military activity near Iran's borders and many senior officers suspect Washington of pursuing regime change in Tehran.
The Guards operate land and naval units separate to the regular armed forces and stage frequent war games in the Gulf, which separates Iran from its regional rival Saudi Arabia and a U.S. naval base in Bahrain.
Last month, the U.S. Navy said an IRGC vessel fired unguided rockets near the aircraft carrier USS Harry S. Truman in the Strait of Hormuz, a critical shipping route for crude oil that connects the Gulf to the Indian Ocean. Iran denied the vessel had done so.
In April 2015, the Guards seized a container ship belonging to Maersk, one of the world's major shipping lines, in the Gulf because of a legal dispute between the company and Iran. The ship and its 24 crew members were released after 10 days.
The Guards have also seized British servicemen on two occasions, in 2004 and 2007, and a civilian British yacht crew in 2009. On each occasion the sailors were released unharmed after several days.
Iran said the British sailors were released when their government apologized to Iran, but London denied that it had offered any apology.