The U.S. and Vietnam say they have completed a rare underwater search for missing Americans from the Vietnam War.

The USNS Bowditch, a U.S. Navy oceanographic survey ship, took part in the four-week mission which ended Monday. It was only the second time in 103 search missions that a U.S. Navy vessel joined with Vietnamese authorities in the search for missing war dead.

The U.S. embassy in Hanoi said the Bowditch, which is manned by a civilian crew, is ideally suited to searching for wrecked aircraft on the ocean floor. The embassy said hundreds of U.S. aircraft and their crews are still missing in the ocean off Vietnam's coast.

The embassy did not say whether any missing aircraft were located during the latest mission.