Four US service members were killed after a US military refueling aircraft crashed in western Iraq, US Central Command (Centcom) confirmed.

The KC-135 tanker went down around 2 p.m. ET (19:00 GMT) on Thursday during ongoing US operations against Iran, Centcom said. The cause of the crash is under investigation.

Centcom initially said there was no indication the aircraft was brought down by hostile or friendly fire. However, Iran’s military later claimed on state television that an allied group had targeted the plane with a missile, according to BBC reporting.

Two aircraft were involved in the incident, Centcom said, with the second plane landing safely. The names of the service members who died are being withheld for 24 hours until their families are notified. The tanker was carrying 6 US service members

The crash occurred in what the US military described as friendly airspace in Iraq, although the region is known to host pro-Iranian militias.

The KC-135, manufactured by Boeing, is designed to refuel aircraft midair and is a key part of US air operations. The tankers extend the range of fighter jets and bombers and were used extensively during the first Gulf War.

Since the outbreak of the US-Israel war with Iran, seven US soldiers have been confirmed killed in the conflict, according to BBC reporting. The US military has also lost at least four aircraft during the war.

Sources: US Central Command; BBC.