The U.S. Marine Corps declared its version of Lockheed Martin Corp.'s F-35 fighter ready for limited combat operations, a milestone for the Pentagon's costliest weapons program.

"The F-35B's ability to conduct operations from expeditionary airstrips or sea-based carriers provides our nation with its first fifth-generation strike fighter, which will transform the way we fight and win," Gen. Joseph Dunford, the Marine Corps commandant, said Friday in an emailed statement.

The declaration of "initial operational capability" came more than five years later than originally predicted in 2001, when the F-35 program began. Earlier delays resulted from difficulties in reducing the plane's weight, with its propulsion system and with reliability.