Five U.S. servicepeople 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.”

"Today, Jill and I mourn the loss of 5 American service members who died when their aircraft crashed in the Mediterranean Sea during a routine training mission. Our service members put their lives on the line for our country every day," President Joe Biden said in a statement on Sunday. "Jill and I are praying for the families and friends who have lost a precious loved one—a piece of their soul. Our entire nation shares their grief. And, during this weekend when we pause to honor our nation’s veterans, we once more affirm the sacred obligation we bear to those who volunteer to serve our nation as well as their families, caregivers, and survivors."

The military first announced the crash on Saturday and said that the cause is under investigation, but there are no indications of any hostile activity involved. It said on 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,” he 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 which military service the aircraft belonged to. The Air Force has sent additional squadrons to the region and the USS Gerald R. Ford aircraft carrier, which has an array of aircraft on board, has also been operating in the eastern Mediterranean.