WASHINGTON - Barely half of the F-35 strike fighters delivered to the US military and international buyers to date are currently available for flight, the government's program head has said.

According to local media, Mat Winter, director of the F-35 Joint Program Office, recently told reporters at a round-table event that only 51 percent of the 280 operational F-35s are flight-ready.

The F-35 was developed and produced with the strategy called "concurrency," the overlapping of development phases to speed the fielding of the fighter, which critics said introduces the risks that aircraft built in early lots will require modification.

The Pentagon is in negotiation with the aircraft's maker Lockheed Martin Corp. about the prices for the 11th batch of F-35s, and Winter said the defense company is moving slow in production and lacks transparency on costs.