Four of six crew aboard a US military refuelling aircraft have died after it crashed in western Iraq, US Central Command (Centcom) says.

Rescue efforts continue after the loss of the KC-135, it said, having earlier said neither hostile nor friendly fire were involved in the downing of the aircraft.

The tanker had been involved in ongoing US operations against Iran, and was one of two aircraft involved in the incident. The second landed safely.

The Boeing-manufactured aircraft are capable of refuelling planes midair and typically play a major role in US military operations. They were used extensively in the first Gulf War to extend the range of fighter jets and bombers.

Centcom said the incident occurred around 14:00 ET (19:00 GMT) on Thursday and that the circumstances of the crash were now under investigation.

It added that the identities of the personnel who had been killed were being withheld for 24 hours so their next of kin could be notified.

The KC-135 usually has a crew of at least a pilot, a co-pilot and a boom operator responsible for controlling the refuelling arm of the aircraft.

Centcom earlier described the crash as happening over friendly airspace - but this is a region of Iraq where pro-Iranian militias operate. Iran's military claimed on state TV that an allied group had targeted the plane with a missile.

Since the outbreak of the US-Israel war with Iran, seven US soldiers have been confirmed as having been killed in the conflict.

The US military has now lost at least four aircraft during the current war.

Earlier this month, three F-15s were shot down in "an apparent friendly fire incident" over Kuwait, officials said. All six crew members were able to safely eject.

Boeing manufactured the KC-135 Stratotanker for the US military in the 1950s and early 1960s.

It has been a backbone to the US military's air refuelling fleet, and allow combat aircraft to carry out longer missions without needing to land. (BBC)