MELBOURNE, Australia – Singapore is looking at ways dispose of its fleet of Boeing KC-135R tanker aircraft as deliveries of its new Airbus tankers continue.

In response to queries from Defense News about what the nation was going to do with the four Republic of Singapore Air Force (RSAF) KC-135R planes, the defense ministry said that “the disposal plans are being worked out and relevant approvals will be sought as required.”

Defense News understands from various sources at least one potential operator has expressed an interest in the Singaporean aircraft with an eye on potentially buying them. As the KC-135s were built in the United States, any disposal method will need the approval of the U.S. government before proceeding.

The ministry added that “the KC-135R will be progressively drawn down as the A330 Multi-Role Tanker Transport is operationalized,” though multiple sources have told Defense News that the fleet has stopped flying since late June with a retirement ceremony for the type having been held in early July.

Singapore acquired four former U.S. Air Force KC-135As stored at the aircraft boneyard in Arizona in the mid-1990s, bringing them up to KC-135R standard with more powerful engines along with a new avionics suite and glass cockpit. The four aircraft, which were originally built between 1959 and 1963, entered service with the Singaporean military in 1999.

These developments comes as four of the six Airbus A330 multi role tanker transports Singapore ordered in 2014 to replace the KC-135Rs have arrived home after having been handed over to the RSAF from the manufacturer.

In addition to Singapore, the KC-135 is still being operated by the U.S. Air Force, France, Turkey and Chile.