Tom Vanden Brook, and Jim Michaels

USA TODAY

WASHINGTON — Iran detained 10 sailors aboard two small U.S. Navy boats on Tuesday, but assured U.S. officials the crew and vessels would be promptly returned.

The two riverine boats were en route from Kuwait to Bahrain when the military lost contact with the vessels, a U.S. military official told USA TODAY. The U.S. is communicating with Iranian officials, who gave assurances of the sailors' safety, the official said.

But the semi-official Iranian news agency FARS reported that Iranian Revolutionary Guards arrested 10 U.S. troops for trespassing in Iranian waters. Nine men and one woman were detained. The vessels carried three .50-caliber machine guns, the agency reported.

Secretary of State John Kerry spoke to his Iranian counterpart, Foreign Minister Mohammed Javad Zarif, who promised the two ships would be allowed to continue their voyage, likely at dawn Wednesday local time, according to a senior U.S. official.

The two officials spoke on condition of anonymity because they were not authorized to discuss the incident publicly.

The boats drifted into Iranian coastal waters in the vicinity of Farsi Island in the north Persian Gulf, apparently after one experienced mechanical problems and the other attempted to render aid. It was not clear how far into Iranian waters they traveled, but the move was unintentional, the senior official said.

White House Deputy National Security Adviser Ben Rhodes said the administration was aware of the incident. “We are working to resolve the situation such that obviously the U.S. personnel are returned to their normal deployment.”

The incident could be a key test for Iran-U.S. relations following a nuclear deal in which the United States and other world powers agreed to lift international sanctions in return for Iran reducing its nuclear program. Relief from the sanctions could begin as early as this week. Kerry and Zarif developed a close relationship during the long negotiations.

While the U.S. expressed confidence the incident would be resolved peacefully, Iran's elite paramilitary forces could continue to provoke the West by exploiting the situation.

Sanctions relief for Iran could come this week

The Iranian seizure is the latest flare-up in an increasingly tense relationship. The U.S. military released a video Saturday showing what it says is an Iranian military vessel firing several unguided rockets near the aircraft carrier USS Harry S. Truman, other warships and commercial craft.

That incident occurred Dec. 26 in the Strait of Hormuz. The images show what appears to be an Iranian Revolutionary Guards vessel firing rockets from about 1,500 yards, or less than a mile.

Video shows Iranian rockets launched near U.S. carrier

Iran’s aggression will take center stage Wednesday at the Supreme Court in Washington. The justices will hear arguments that hundreds of victims of Iranian-sponsored terrorism and surviving family members should win access to about $2 billion in frozen assets of Iran's central bank.

Among them are relatives of 173 of the 241 servicemen killed in the 1983 bombing of a Marine Corps barracks in Beirut. It was carried out by the Lebanese terrorist group Hezbollah, but federal courts in the United States held Iran responsible. Now the high court must decide if Congress overstepped its bounds by passing a law specifically designed to resolve the standoff over the frozen assets.

Contributing: Gregory Korte and Richard Wolf, USA TODAY, and Navy Times.