Iranian Revolutionary Guards have detained two U.S. boats with 10 crew aboard after the vessels "illegally" entered Iranian territorial waters in the Gulf, the semi-official Fars news agency said early on Wednesday.
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"These two boats were seized by Revolutionary Guard naval forces near Farsi Island 2 km inside Iranian territorial waters," Fars reported. It gave no further detail.
The White House said on Tuesday that it was aware of the situation in which two Navy boats were held in Iranian custody and is working to get U.S. personnel returned.
"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.
Two U.S. officials told Reuters that it was unlikely the sailors would be released overnight, and a defense official said that plans were in place to return them to the U.S. Navy early Wednesday morning.
Earlier, U.S. defense officials said the sailors - nine men and one woman - had been on two riverine patrol boats when they were taken into Iranian custody. The United States says the ships were en route from Kuwait to Bahrain at the time.
A U.S. official said mechanical issues may have disabled one of the boats, leading to a situation in which both ships drifted inadvertently into Iranian waters.
Iranian Foreign Minister Mohammad Javad Zarif told U.S. Secretary of State John Kerry on Tuesday that U.S. sailors who ended up in Iranian custody would be allowed to continue their journey promptly, a senior Obama administration official said on condition of anonymity.
Kerry and Zarif were two of the main negotiators in the July 14 agreement under which Tehran agreed to restrain its nuclear program in return for relief from economic sanctions.
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. Obama, a Democrat, made the Iran accord a centerpiece of his foreign policy, and Republicans vying to succeed him have assailed him over the deal.
Tuesday's event 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 December 26 near warships including the aircraft carrier USS Harry S Truman in the Strait of Hormuz. Iran denied the vessel had done so.