While details are sparse, two U.S. Navy riverine command boats have been “taken into custody” by the government of Iran, the Pentagon said Tuesday.

An Iranian state-run news outlet reported that 10 U.S. sailors had been “arrested” even though Iranian and U.S. officials said that none of the sailors were harmed and that they would be released promptly. It is unclear how the boats ended up in Iranian hands, though officials have stated that one of the boats broke down very close to Iranian territorial waters and after drifting for a short time both were picked up by Iranian forces.

Riverine command boats or RCBs, are actually Swedish CB-90s and are a type of fast attack craft. In a number of pictures released by the U.S. Navy, the boats are outfitted with a number of light, medium and heavy weapons including .50 caliber heavy machine guns and GAU-19 miniguns. Approximately 50 feet long, the CB-90s can reach speeds up to 40 knots on the water, according to the Swedish Navy.

RCB’s can carry contingents of infantry and special operation forces and are often crewed by sailors in Riverine squadrons, known by some as River Rats. The riverine force came of age in the Vietnam War in what was then known as the Brown Water Navy. In the 1960s and early 1970s boats such as Patrol Boat, River (from ‘Apocalypse Now’ fame) and Swift Boats were the River Rats vessels of choice.

Ten sailors 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. Deborah Lutterbeck reports. (Reuters)