Four of six members of a US military refuelling aircraft's crew have been confirmed dead 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.

The US military command unit 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.

Thursday's crash brings the official US military death toll in the US-Israel war with Iran, which began a fortnight ago, to 11. 

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.

Separately, Israel's military said on Friday that it had launched a fresh "wide-scale wave of strikes" targeting Iranian infrastructure across Tehran.

Explosions were later reported in and near the Iranian capital.

It came after the US loosened sanctions on other countries buying Russian oil and petroleum already loaded on vessels at sea to curb the economic impact of the US-Israel war with Iran.

Oil prices climbed above $100 a barrel again on Thursday, while stock markets fell after three more cargo vessels were hit in the Gulf and Iran's new supreme leader vowed to keep blocking the Strait of Hormuz, a vital shipping lane.

US President Donald Trump has indicated that the war could end "very soon" since the price of oil shot up, while also suggesting the US could "go further".

France's military announced on Friday that a drone had killed a French soldier, who it identified as Staff Sgt Arnaud Frion, and wounded six others at a Kurdish military base in northern Iraq yesterday.

It said the soldiers had been stationed at the base to train Iraqi units.