Japan's defence chief said Wednesday the country may cancel its $4.7 billion order for the US-built F-35 stealth jet if Washington fails to stick to the proposed price and deadlines.
Defence Minister Naoki Tanaka said a formal contract for the initial four units which Japan wants by March 2017, out of a total of 42 jets, was expected to be signed before this summer.
But, he told parliament: "If the situation comes to the point where (the US) cannot work out the proposal by that time, we will have concerns for Japan's defence capability.
"We would have to look at either cancelling the contract or opting for another model."
Japan in December chose Lockheed Martin's F-35 stealth jet for its next-generation mainstay fighter over two other jets -- the Boeing-made F/A-18 Super Hornet and the Eurofighter Typhoon.
Tokyo has opted for the jet to replace its ageing fleet of F-4 fighters, despite a series of technical setbacks and fears that the as-yet unproven F-35 might be badly delayed.
With a price tag of around $113 million per jet, the F-35 is the most expensive weapons programme in Pentagon history and has been plagued by cost overruns and technical delays.
Earlier this month, Japan's Defence Ministry sent a letter to the Pentagon asking it to keep the proposed price and deadlines for delivery.
Japan, which places its security alliance with the United States at the centre of its foreign policy, has long depended on US manufacturers for its military hardware.