Japan will buy four US-made F-35 stealth jets despite a sharply higher price tag, it said Friday, in Tokyo's first confirmed order for the next-generation aircraft which has been plagued by delays.

An official from the defence ministry said Japan would now pay 9.6 billion yen ($120 million) per aircraft, up from the $110 million originally earmarked.

He added US officials had said the price rise was unavoidable and Tokyo accepted the situation.

"We learned that the reason for the price hike is because the United States decided to postpone its domestic procurement of 179 aircraft in the next five years due to its tight budget," the official said, speaking on condition of anonymity.

"We accepted it as it is understandable. It would be hard for them to offer a lower price only to Japan, given that the jet was co-developed by nine countries," he said.

The defence ministry last year picked the Lockheed Martin jet to replace its ageing fleet of F-4s over the rival Boeing-made F/A-18 Super Hornet and the Eurofighter Typhoon, despite a series of technical setbacks.

The F-35, co-developed with British defence giant BAE Systems, was the most expensive among the three candidates.

In February, Japan's then defence minister Naoki Tanaka threatened to cancel the whole $4.7 billion, 42-jet order amid continued cost and time slippages.

On Friday, Tokyo signed a Letter of Offer and Acceptance for four fighter jets at $120 million each, along with two simulators and other accessories for a total cost of 60 billion yen.

The planned purchase of the other 38 jets has yet to be formally confirmed.

The F-35 is the most expensive weapons programme in Pentagon history and has been plagued by cost overruns and technical delays.

Last year a leaked memo revealed an array of problems exposed by flight tests, including with the landing gear and issues over airframe fatigue and vibration.

The United States touts the F-35 as a technological wonder that will slip past enemy radar and allow allied forces to keep operating in the skies alongside US warplanes.

But the programme's costs have skyrocketed just as governments around the world are facing severe budget pressures and austerity measures.

Italy has reduced its planned purchase from 131 to 90 aircraft and lawmakers in the Netherlands voted to limit their order to two, while an initial plan for 138 aircraft in Britain has been dropped without a firm number announced.