BERLIN — Five U.S. service members were killed when a military helicopter crashed over the eastern Mediterranean Sea during a training mission, U.S. officials said Sunday.

The military’s European Command said all five crew members on board were killed when the aircraft went down “during a routine air refueling mission as part of military training.”

The military first announced the crash Saturday and said the cause is under investigation, but there are no indications of any hostile activity involved. It said Sunday that “search and rescue efforts began immediately, including nearby U.S. military aircraft and ships.”

U.S. Defense Secretary Lloyd Austin said in a statement that “we mourn the tragic loss of five U.S. service members during a training accident in the Mediterranean Sea early Saturday morning.”

“While we continue to gather more information about this deadly crash, it is another stark reminder that the brave men and women who defend our great nation put their lives on the line each and every day to keep our country safe,” Austin said.

European Command said that out of respect for the families of the service members and in line with Department of Defense policy, the identities of the crew members are being withheld for 24 hours until the families of those killed have been notified.

It wasn’t immediately clear to which military service the aircraft belonged. The Air Force has sent additional squadrons to the region, while the USS Gerald R. Ford aircraft carrier, which has an array of aircraft on board, has also been operating in the eastern Mediterranean.