WASHINGTON (Reuters) - The U.S. Defense Department said it will sign a contract with Lockheed Martin Corp on Friday for a fifth group of F-35 Joint Strike Fighter jets, a long-awaited deal that moves forward the most costly weapons program in U.S. history.

The Pentagon also said it expects to reach an agreement soon with Lockheed on early funding for a sixth group of F-35s, a step that could help reduce a potential $1.1 billion liability the weapons maker faced from work it had already done on the jets without a signed contract.

(Reporting by Andrea Shalal-Esa; Editing by Jeffrey Benkoe)