Lockheed Chief Executive Robert Stevens told a Morgan Stanley investor conference that negotiations with the US Defense Department were ongoing, but an agreement could be reached by the end of the year.

"I think it's possible that we can do that," Stevens said.

The contract discussions between Lockheed and the Pentagon began in December 2011 and have dragged on much longer than initially expected, with government officials insisting on extremely detailed pricing data.

A deal had appeared near during the company's second quarter earnings call, when Chief Financial Officer Bruce Tanner told analysts that he expected a deal "in the not-too-distant" future.

Stevens gave no further details on the contract talks, but said Lockheed was making progress on testing the aircraft, and was working to resolve lingering challenges on the sophisticated helmet that will be used by F-35 pilots and the complex software required for the program.

He said Lockheed had delivered 16 of the 30 jets it was due to deliver this year, and was continuing to press for additional international orders to help drive down the overall cost of the program.