Iran Holding 2 Navy Boats in Persian Gulf

US Navy riverine boats are seen in this 2012 Defense Department photo.

Crew members from two small U.S. Navy boats are being held by Iran.

Pentagon officials said Tuesday that Tehran has promised them that the crew and boats will be "promptly" returned.

A senior U.S. defense official told VOA Tuesday there are 10 U.S. sailors on the two boats. A U.S. official confirmed there were nine men and one woman.

The boats were traveling between Kuwait and Bahrain when the U.S. lost contact with them.

CNN reported that one of the boats might have had mechanical problems.

Washington has been in communication with the Iranians, who have promised to "promptly allow them to continue on their journey," the senior defense official said.

A senior administration official says Secretary of State John Kerry spoke by telephone at midday Tuesday with his Iranian counterpart, Mohammed Javad Zarif, shortly after the incident occurred.

U.S. officials said that the incident happened in the Persian Gulf near Farsi Island.

The news comes less than a month after U.S. officials accused Iran of launching a rocket test near U.S. boats passing through the Strait of Hormuz.​

I'm Jim Tedder.

UPDATE without audio:

Farsi Island is home to an Iranian Revolutionary Guard Corps naval base. That may be why the sailors were quickly detained, said Matthew Kroenig, a senior fellow in the Atlantic Council’s Brent Scowcroft Center on International Security, to VOA Tuesday.

“Most countries would do the same thing if foreign sailors came that close to a naval base,” Kroenig explained.

It also came just hours before President Barack Obama was to deliver his final State of the Union address to Congress and the public. Part of the address is expected to address concerns of Congress over a nuclear deal between Tehran and Western powers. That deal was reached last year.

The deal is set to curb Iran's nuclear program. In return, Western governments will lift economic sanctions against the Islamic Republic.