Four service members were killed when their refueling aircraft “went down” in friendly airspace in western Iraq, according to U.S. Central Command.

“Four of six crew members on board the aircraft have been confirmed deceased as rescue efforts continue,” CENTCOM said Friday.

The KC-135 aircraft went down at approximately 2 p.m. ET when two aircraft were involved in “an incident,” CENTCOM said in a brief statement Thursday, confirming that “one of the aircraft went down in western Iraq, and the second landed safely.”

Gen. Dan Caine addressed the crashed refueling plane, saying the incident is being treated as an active rescue and recovery mission. 

“The incident occurred over friendly territory in western Iraq while the crew was on a combat mission, and again, was not the result, as CENTCOM has said, was not the result of hostile or friendly fire,” Caine said Friday. “We’re still treating this as an active rescue and recovery operation, as CENTCOM announced this morning, four airmen have been recovered, and the Air Force and US Central Command will provide updates as information becomes available.”

The other aircraft involved was also a KC-135 tanker, according to a U.S. official.

The circumstances of the incident are currently under investigation and the identities of the service members who died in the incident are being withheld until 24 hours after next of kin have been notified, officials said.

KC-135 aircraft are not equipped with parachutes and do not have ejection seats, which are primarily in fighter aircraft, officials have told ABC News.

Passengers and crew members of KC-135s instead are trained on how to exit the aircraft when it is on land or on water, officials said.

According to a 2008 Air Force profile of the tanker crews, the move to get rid of parachutes was made because the tankers “seldom have mishaps, and the likelihood a KC-135 crew member would ever need to use a parachute is extremely low.”

This is a developing story. Please check back for updates.