Military officials did not specify where the helicopter was flying from, but the United States has deployed a carrier strike group to the Mediterranean as part of efforts to prevent the Israel-Hamas war from spiraling into a regional conflict.

"During a routine air refueling mission as part of military training, a US military aircraft carrying five service members suffered a mishap and crashed into the Mediterranean Sea. All five of the service members onboard the aircraft were killed," the US European Command (EUCOM) said in a statement.

President Joe Biden, in a statement from the White House, paid tribute to the victims, who died on a weekend when Americans were honoring military veterans.

"Our service members put their lives on the line for our country every day," Biden said. "They willingly take risks to keep the American people safe and secure. And their daily bravery and selflessness is an enduring testament to what is best in our nation."

Defense Secretary Lloyd Austin also offered his condolences in a statement that identified the crashed aircraft as a helicopter.

While EUCOM initially said the crash occurred on November 10, Austin said the helicopter went down on Saturday morning.

- Israel-Hamas war -

Washington rushed military support to Israel and bolstered its forces in the region -- including with the USS Gerald R. Ford aircraft carrier and other warships -- after the Hamas militant group carried out a shock cross-border attack from Gaza on October 7 that Israeli officials say killed about 1,200 people, mostly civilians.

Israel's military responded with a relentless air, land and naval assault on Gaza that the territory's health ministry said has left more than 11,000 people dead.

US forces in the region have faced a spike in attacks linked to the conflict in recent weeks and have been targeted more than 40 times since mid-October, leaving dozens of American personnel with minor injuries.

Washington has blamed Tehran-backed militias for the violence and has carried out three strikes against Iran-linked sites in Syria -- two on October 26 and one on Wednesday.

There have been multiple other crashes of US military aircraft in recent years, including an F-35 stealth warplane that went down in September, with the pilot able to eject.

In March, two US Army helicopters crashed during a nighttime training mission in Kentucky, killing all nine soldiers on board.

And four US Marines were killed during NATO exercises in Norway last year when their V-22B Osprey aircraft went down.