A US military refueling aircraft crashed in western Iraq on Thursday, in an incident US Central Command said involved another aircraft but was not the result of hostile or friendly fire.
The United States has deployed a large number of aircraft into the Middle East to take part in operations against Iran and the incident highlights the risk of operations, even over friendly skies.
In a statement, US Central Command said it was carrying out rescue efforts after the US KC-135 refueling aircraft went down. The second aircraft landed safely.
"The incident occurred in friendly airspace during Operation Epic Fury," the statement said, using the military name of the US operation against Iran.
A US official, speaking on the condition of anonymity, said the other aircraft involved in the incident was also a KC-135 and the one that crashed had as many as six service members on board.
Later, the Central Command said four of the six crew members on board were confirmed dead.
The Islamic Resistance in Iraq, an umbrella group of Iran-backed armed factions, claimed responsibility for downing the US military refueling aircraft.
The group said in a statement it had shot down the KC-135 aircraft "in defense of our country's sovereignty and airspace".
The KC-135, built by Boeing in the 1950s and early 1960s, has served as the backbone of the US military's air refueling fleet and is critical to allow aircraft to carry out missions without having to land.
Since the US and Israel started carrying out strikes against Iran on February 28, seven US troops have been killed. The United States has carried out strikes against more than 6,000 targets in Iran.
Reuters reported on Tuesday that as many as 150 US troops have been wounded in the US-Israeli war on Iran. News of the crash comes the same day two U.S. sailors were injured after the USS Gerald Ford suffered a non-combat-related fire on board.
So far the war has killed more than 2,000 people, including almost 700 in Lebanon.
