The Pentagon’s F-35 warplane is giving U.S. allies in Asia a headache as they look to replace ageing jets with a cutting edge aircraft now likely to be at least seven years late in offering a strategic deterrent to China.

The $400 billion weapons project has suffered technical faults, delays, cost overruns and now U.S. budget cuts that could force Washington to scale back its own purchases.

At the same time, China’s soaring defence spending is rapidly eroding the advantage in technology, particularly in air power, that Washington and some of its regional allies have had over the People’s Liberation Army (PLA) since the 1950s.

China is also flight testing two stealth fighters, the J-20 and the J-31, although they are not expected to enter service until the end of the decade at the earliest, military aviation experts said.

“It’s an open question as to how advanced and sophisticated they actually are,” said Andrew Davies, a senior strategy analyst at the Australian Strategic Policy Institute, referring to the Chinese fighters.

“But having said that, they make life more difficult for existing types, so the F-35 becomes more important.”

The F-35 is the costliest weapons programme in history. Lockheed Martin Corp is the prime contractor. Pratt & Whitney, a unit of United Technologies Corp, makes the engine.

Australia has ordered 100 F-35s, although defence analysts say it might buy only 50 to 70 given Canberra is expected to decide in June to double its fleet of 24 Boeing Co F/A-18 Super Hornets. That would prevent a frontline gap until the F-35 is delivered later in the decade.

Japan said it was not changing its plan to buy 42 planes while South Korea is expected to choose the F-35 as the winner of a 60-jet competition to be decided this summer.