Five U.S. Army Special Operations forces have died in a helicopter crash in the eastern Mediterranean Sea, American officials said on Sunday.

The troops were crew members of an MH-60 Black Hawk helicopter that was on a refueling training mission late Friday when the aircraft crashed off the coast of Cyprus, three U.S. officials said, speaking on the condition of anonymity to discuss operational details. The crash is under investigation, they said.

The Pentagon has quietly dispatched to Cyprus commando teams from the Joint Special Operations Command, including the Army’s Delta Force and the Navy’s SEAL Team 6, to stand by in case they are needed to help evacuate American citizens from the region.

The commandos are also trained in hostage rescue operations. About a dozen or so American hostages were seized when Hamas attacked Israel on Oct. 7, but Biden administration officials have indicated they have no plans to put American boots on the ground in the densely populated Gaza Strip, where the Israel military is now conducting major ground operations.

The helicopter crew members who died were members of the Army’s elite 160th Special Operations Aviation Regiment, known as the Night Stalkers, and are among the aviators assigned to ferry the commandos on clandestine missions.

An American aircraft carrier, the Gerald R. Ford, is also operating in the eastern Mediterranean off the coast of Israel, in what the Biden administration has said is a deterrent to Iran and its proxies in the region to widen the Gaza war.

In a statement on Sunday, the military’s European Command acknowledged the deaths of the five service members in what it said was “a routine air refueling mission,” but released no other information about the crash, the crew or its unit in a reflection of the secrecy surrounding the unit’s mission.

Defense Secretary Lloyd J. Austin III said in a statement on Sunday, “We mourn the tragic loss of five U.S. service members during a training accident in the Mediterranean Sea.”

President Biden, noting that it was Veterans Day weekend, said: “Our service members put their lives on the line for our country every day. They willingly take risks to keep the American people safe and secure.”