Two Marine Corps transport helicopters crashed Friday off the coast of Djibouti, the Pentagon said.
Two of the 12 crew members were rescued. The status of the other 10 was not immediately known, officials said.
A search-and-rescue mission by troops from the United States, Djibouti and France was under way, according to a statement issued by Combined Joint Task Force Horn of Africa, a U.S.-led military force headquartered at Camp Lemonier, a French military base in Djibouti.
The helicopters were on a nighttime training mission at the time of the crash, whose cause had not been determined Friday night.
A CH-53E helicopter normally operates with a crew of between three and eight people.
At the Pentagon, a spokesman, Lt. Cmdr. Joe Carpenter, said there was no indication of hostile fire.
Members of the Djiboutian military notified U.S. officials at about 5:30 p.m. local time (9:30 a.m. EST) that the helicopters had crashed in the Gulf of Aden not far from the Djiboutian coastal town of Ras Siyyan.
Djiboutian military members who were near the impact site were able to rescue two injured crew members, according to the Combined Joint Task Force Horn of Africa statement. The two were transported to Camp Lemonier and were listed in stable condition.
Djibouti is located between Somalia and Ethiopia.