Iran's Revolutionary Guard said on Wednesday it had released 10 American sailors detained a day earlier, according to the Guards' statement broadcast on state television.
Iran detained the sailors aboard two U.S. Navy patrol boats in the Gulf on Tuesday in an incident that rattled nerves days ahead of the expected implementation of a landmark nuclear accord between Tehran and world powers.
Gen. Ali Fadavi, Navy chief of Iran's powerful Revolutionary Guard, was quoted Wednesday on Iran's state TV as saying: "Investigation shows that entry of American sailors into Iran's territorial waters was due to mechanical problems in their navigation system and that issue is being resolved."
On Tuesday, White House spokesman Josh Earnest told CNN, "We have received assurances from the Iranians that our sailors are safe and that they will be allowed to continue their journey promptly."
Both sides appeared eager not to let the incident escalate. It came at a delicate time for U.S.-Iranian relations, after Iran and six world powers forged a landmark nuclear accord last July.
Formal implementation of the accord could begin in days following steps Iran agreed to take to curb its nuclear activities.
A senior U.S. defense official said the U.S. had lost contact earlier in the day with two small craft en route to Bahrain from Kuwait. U.S. officials told Reuters it was unclear how or if the boats became disabled.
Iranian Foreign Minister Mohammad Javad Zarif assured U.S. Secretary of State John Kerry that the U.S. sailors would be allowed to continue their journey promptly, another U.S. official said.
Iran's semi-official Fars news agency said Iranian Revolutionary Guards had detained the vessels after they "illegally" entered Iranian territorial waters in the Gulf. The two boats were seized near Farsi Island 1.2 miles inside Iranian territorial waters, it said.
Officials from Iran and the U.S. are negotiating to free the crew, Fars reported. U.S. defense officials said nine men and one woman were aboard the two vessels seized.
News of the incident broke as U.S. President Barack Obama prepared to make his final State of the Union address to the U.S. Congress. He is due to leave office in January 2017.
The sailors were on board two riverine patrol boats, one of the officials said. Riverine boats are 38-foot-long, high-speed patrol boats used by the U.S. Navy and Marines to patrol rivers and littoral waters.
It was the latest reported incident between U.S. and Iranian forces in the Gulf in recent weeks.
The U.S. Navy said late last month that an Iranian Revolutionary Guards vessel fired unguided rockets on Dec. 26 near warships including the aircraft carrier USS Harry S. Truman in the Strait of Hormuz. Iran denied the vessel had done so.
Previous Iranian seizures involved British sailors and marines.
In June 2004, Iran arrested six Royal Marines and two naval personnel — part of a U.S.-led force in Iraq — for straying into its waters, stirring diplomatic tensions between the two. Following negotiations the eight were freed three days later.
In March 2007, Iranian forces seized 15 British servicemen — eight Royal Navy sailors and seven marines — in the mouth of the Shatt al-Arab waterway that separates Iran and Iraq, triggering a diplomatic crisis at a time of heightened tensions over Tehran's nuclear ambitions. They were held for 13 days.
In November 2009, Iranian naval vessels detained five Britons on a racing yacht en route from Bahrain to Dubai. They were released a week later.