Ten sailors aboard two U.S. Navy boats were seized by Iran in the Gulf on Tuesday, and Tehran told the United States the crew members would be promptly returned, U.S. officials said.
"We have received assurances from the Iranians that our sailors are safe and that they will be allowed to continue their journey promptly," White House spokesman Josh Earnest told CNN.
A senior U.S. defense official said the U.S. had lost contact earlier in the day with two small craft en route from Kuwait to Bahrain. 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.
While both sides appeared eager not to let the incident escalate further, it came at a delicate time for U.S.-Iranian relations. 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.
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 2 km (1.2 miles) inside Iranian territorial waters, it said.
Officials from Iran and the United States 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.
They 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. (Additional reporting by Arshad Mohammed and Jonathan Landay in Washington and Parisa Hafezi and William Maclean in Dubai; Writing by Sam Wilkin and Tom Brown; Editing by David Alexander, Warren Strobel and Howard Goller)