Boeing's KC-46 aerial refueling tanker conducts receiver compatibility tests with a U.S. Air Force C-17 Globemaster III from Joint Base Lewis-McChord, in Seattle, Washington, U.S., July 12, 2016. Christopher Okula/ U.S. Air Force/Handout via REUTERS/File Photo

(Reuters) - The U.S. Air Force said there would be a further delay in taking delivery of Boeing Co’s KC-46 aerial refueling aircraft, as the planemaker tries to get airworthiness certifications and complete a flight test program.

The U.S. Air Force assessment predicts first aircraft delivery beyond Boeing’s forecast into late spring of 2018, according to a statement by the U.S. Air Force.

Boeing had previously forecast the aircraft to be delivered by September this year, and now expects first aircraft delivery by December 2017.

The top issues slowing progress are achieving the Federal Aviation Administration (FAA) airworthiness certifications and completing the flight test program, the U.S. Air Force said.

Once Boeing gets the remaining design approvals from the FAA, it expects testing to proceed at a faster pace, according to the statement.

The aircraft delivery delay is not expected to result in additional costs to the taxpayer, the U.S. Air Force said.