The US Air Force said Wednesday its eye-wateringly expensive F-35 stealth jet is superior to the cheaper F-16 despite reports from a test pilot that suggested otherwise.

The F-35, manufactured by defense giant Lockheed Martin in part to replace the F-16, is the most expensive fighter plane in the history of the US military.

But the $390-billion program is seven years behind schedule and dogged by delays.

Adding to the doubts, a test pilot who flew the plane in January reported the aircraft does not turn or climb fast enough to hit an enemy plane or dodge gunfire, according to a brief cited by the War Is Boring blog.

"The F-35 was at a distinct energy disadvantage," the pilot said, according to War Is Boring.

The test pilot reportedly engaged in a series of maneuvers involving an F-16, according to the site.

But the Air Force said there were several scenarios in which the F-35 proved to be superior.

"There have been numerous occasions where a four-ship of F-35s has engaged a four-ship of F-16s in simulated combat scenarios and the F-35s won each of those encounters because of its sensors, weapons and stealth technology," the Air Force said in a statement.

Major Gen Jeffrey Harrigian, director of the F-35 Integration Office at the Pentagon, said tests would continue on the F-35 and that its design was an improvement on the F-16.

"Both operational and developmental test continues for the F-35. It is too soon to draw any final conclusions on the maneuverability of the aircraft," he said.

"The F-35 is designed to be comparable to current tactical fighters in terms of maneuverability, but the design is optimized for stealth. This will allow it to operate in threat environments where the F-16 could not survive."

F-35 fighter jets are not yet in use by the US military. The Marines are expected to be the first branch of the military to operate the new jet, which could occur over the next several months.