The Pentagon on Saturday released the identities of six US crew members killed during the crash of a refueling aircraft in western Iraq earlier this week, which authorities said was not caused by "hostile fire."

The KC-135 aerial refueling aircraft crashed in western Iraq on Thursday, bringing the number of US troops killed in operations against Iran to at least 13. A second aircraft involved in the operation landed safely.

The Pentagon said the six members killed in the crash were: John Klinner, 33, of Auburn, Alabama; Ariana Savino, 31, of Covington, Washington; Ashley Pruitt, 34, of Bardstown, Kentucky; Seth Koval, 38, of Mooresville, Indiana; Curtis Angst, 30, of Wilmington, Ohio; and Tyler Simmons, 28, of Columbus, Ohio.

The first three were members of the US Air Force, while the latter three were stationed with the US Air National Guard.

The crash remains under investigation, Pentagon officials said, but US Central Command previously stated that "the loss of the aircraft was not due to hostile fire or friendly fire."

The Islamic Resistance in Iraq, which is a loose alliance of Iran-backed Iraqi factions, claimed to have downed a KC-135. They also said they had targeted another plane that escaped.

Since the start of the Middle East war, the alliance has been claiming daily attacks on US interests in Iraq and across the region.

The KC-135 crash is at least the fourth US military aircraft lost during the war, after three F-15s were shot down by friendly fire over Kuwait.

KC-135s, which have been in operation for more than 60 years, generally have a crew of three -- a pilot, a copilot and a third who operates the boom used to refuel other aircraft, according to the US Air Force.

But some KC-135 missions require a navigator, and the aircraft can carry up to 37 passengers, an Air Force factsheet said.

Early in the war -- which began on February 28 -- Kuwaiti forces mistakenly downed three American F-15E fighters, but all six crew members were able to eject, according to CENTCOM.

That incident occurred during combat including "attacks from Iranian aircraft, ballistic missiles, and drones," the military command said at the time.