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  Iran ians 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.

Iran ian 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.- Iran ian 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  Iran ian Revolutionary Guards had detained the vessels after they "illegally" entered  Iran ian territorial waters in the Gulf. The two boats were seized near Farsi Island 2 km (1.2 miles) inside  Iran ian territorial waters, it said.

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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  Iran ian forces in the Gulf in recent weeks.

The U.S. Navy said late last month that an  Iran ian 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  Iran ian 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,  Iran ian 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,  Iran ian naval vessels detained five Britons on a racing yacht en route from Bahrain to Dubai. They were released a week later.