After experiencing mechanical problems and drifting into Iranian waters on Tuesday, the Associated Press reports, two small U.S. Navy boats and their crews were detained and held by Iranian authorities. American officials say Tehran assures them the ten sailors will be returned safely.

According to Pentagon spokesman Peter Cook, Navy commanders lost contact with the two riverine boats—one of which ultimately ran aground, the AP reports—as they moved between Kuwait and Bahrain. The incident took place near Farsi Island, an Iranian island in the Persian Gulf.

“We have been in contact with Iran and have received assurances that the crew and the vessels will be returned promptly,” Cook said.

One Navy official in Washington told the New York Times that the two boats had failed to arrive at a scheduled rendezvous with another ship to refuel. How contact was lost in the first place remains unclear.

From the Times:

The semifofficial Fars news agency in Iran said that the boats had illegally traveled more than a mile into Iranian territorial waters near Farsi Island, in the Persian Gulf. It said that forces of the Islamic Revolutionary Guards Navy had confiscated GPS equipment, which would “prove that the American ships where ‘snooping’ around in Iranian waters.”

Tabnak, a semiofficial Iranian website, said that 10 American service members had been arrested, but that Iran had assured the United States they would be released immediately.

Late last month, Iran, which is due to fulfill the terms of the arms deal it agreed to last summer in just a few days, initiated a rocket test near U.S. warships in the Strait of Hormuz.

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