WASHINGTON — Although Boeing maintains it can deliver the first KC-46 by the end of the year, the Air Force now believes it will not accept the new tanker until late spring of 2018, the service announced Thursday.

Either way, the first delivery — originally scheduled for September — will be delayed at least a couple months.

"The top issues slowing progress are achieving the FAA airworthiness certifications and completing the flight test program," the Air Force said in a written statement issued days after the annual schedule risk assessment concluded. "Once Boeing receives the remaining design approvals from the FAA, they expect testing to proceed on a faster pace."

The FAA, or Federal Aviation Administration, is an aerospace regulatory agency of the United States government.

At this time, the Air Force hasn't altered its expectation that Boeing will be able to meet the October 2018 deadline for required assets available, or RAA, which call for the company to deliver 18 tankers and nine refueling pods. However, the service said it would have greater confidence in the schedule in July, when Boeing secures final design approvals from the FAA.

The announcement of a projected delay is no surprise. The Air Force has been telegraphing for weeks that the risk assessment would conclude that Boeing would need more time to complete key milestones.

On Wednesday, Lt. Gen. Arnold Bunch, the service’s top uniformed acquisition official, told reporters that the Air Force was briefing lawmakers and would announce the September 2017 goal for first delivery would not be met, nor would it be likely that Boeing delivers the first KC-46 by the end of the year.

"Boeing is still committed, they're putting resources [into it] and their drive is to get it done faster than what we believe they can," he said. "We're all for that. We would welcome that, and we will provide them the resources we can to make that happen."

He added that the service continues to explore options with Boeing on whether the pace of delivery could be accelerated to meet the current RAA date.

In an emailed statement, Boeing spokesman Charles Ramey acknowledges risk in the KC-46 test schedule. However, he asserted that the company can still deliver the first plane in late 2017.

"We expect to be much more efficient during the second half of testing now that the design has stabilized. The majority of developmental testing is complete, which reduces risk to the certification and verification testing. Additionally, we recently added a sixth aircraft (the second [low-rate initial production] plane) to the flight test program, which will help with test efficiency and improve our ability to complete ground and flight test points going forward," he said. "We are not discovering any new technical risk."