Iran has been holding two small Navy boats that drifted into Iranian waters due to mechanical issues. Ten U.S. Navy sailors are on board, collectively. Tehran has assured the Pentagon that they will be returned.

Peter Cook, a spokesman for the Pentagon, said that the boats had been travelling from Kuwait to Bahrain when contact with them was lost, according to the Associated Press.

The mechanical failures occurred near Farsi Island in the Persian Gulf. Due to unknown issues, one of the boats ran aground. Both were detained by Iranian forces, who held them on Farsi Island for some time. Their current whereabouts are nknown.

"We have been in contact with Iran and have received assurances that the crew and the vessels will be returned promptly," Cook said, according to the BBC.

Upon learning of the incident, John Kerry, U.S. secretary of state, called Iranian foreign minister, Javad Zarif. An unnamed official told the Associate Press that Kerry "personally engaged with Zarif on this issue to try to get to this outcome."

Barry McCaffrey, a retired Army Gen. and NBC military analyst, described the seizure of the boats as "very bad news."

"We simply can't allow ground, air or naval units to be seized by a foreign hostile power," McCaffrey said on MSNBC. "I hope they're back real quickly [sic]. This is an affront to our military presence in the Gulf and will unsettle our allies in the region."

The U.S. and Iran are not currently hostile; they have engaged in various diplomatic relations for several months. In July, the two countries, along with others, reached a landmark deal to allow Iran to expand its nuclear power sector. The U.S. will begin buying heavy water, or deuterium oxide, an isotopologue of water, from Iran as soon as the country replaces the reactor core at the Arak nuclear site. Heavy water is needed for cooling in nuclear power plants.

"It is a matter of days, not weeks," said Behrouz Kamalvandi of Iran's atomic agency, according to Agence France-Press.