A 15 billion dollar contract award for a new fleet of combat search-and-rescue helicopters has been delayed until next year due to a "minor" change in the terms of the bid, the US Air Force said Thursday.

It was the second time in recent weeks that a major air force acquisition program has been left for the next administration to decide, following last month's cancellation of a 35 billion dollar contract for a new generation of air refueling tanker planes.

Sue Payton, the service's assistant secretary for acquisition, ordered "a minor amendment to the request for proposal" to the so-called CSAR-X contract, which will be released soon, the air force said in a statement.

"The purpose of the amendment is to further clarify how Air Force officials will make the source selection decision," it said, adding that it would result in a "minor delay" to the award of the contract.

Lieutenant Colonel Ann Stefanek, an air force spokeswoman, said the delay would push back a decision on the winner from December 2008 "into the new year, but we don't know how far."

The so-called CSAR-X contract is for new combat search and rescue helicopters to replace the ageing fleet of HH-60 Pave Hawks.

It was initially awarded to Boeing, but was recompeted in 2006 after an audit upheld protests by rivals Lockheed Martin and Sikorsky.

The Defense Department's inspector general is currently conducting an investigation into how requirements for the new helicopter were developed.

But the air force said the change was unrelated to the investigation, which it said was expected to be completed later this year.

"Air Force officials have full confidence in the CSAR-X source selection process," the statement said.

"The clarification underscores their commitment to a fair and transparent competition, consistent with the secretary of the Air Force's focus on acquisition excellence."