Five United States service members have been killed after the helicopter carrying them on a refuelling training mission crashed into the Mediterranean Sea, the US defence officials say.

US European Command said search and rescue operations were launched immediately following the crash on Friday, and an investigation into its cause was under way.

The US military has deployed two aircraft carriers – the Ford and the Eisenhower – along with supporting ships and dozens of aircraft to the eastern Mediterranean since Hamas’ October 7 attack on Israel to act as a deterrent to ensure the conflict does not expand.

US European Command, which oversees American military operations in Europe and parts of the Middle East, first reported the accident on Saturday.

“We can definitively say that the aircraft sortie was purely related to training and there are no indications of hostile activity,” the European Command said on Saturday.

It did not identify the helicopter involved.

Identities of the victims will not be released until next of kin notifications are completed, the command said.

President Joe Biden expressed condolences in a statement on Sunday.

—AAP