"If the U.S. side makes an unrealistic proposal, we will cancel the purchase or consider (purchasing) other types of aircraft," Japanese Defense Minister Naoki Tanaka told the Kyodo News Agency.
After considering competing aircraft -- Boeing's FA-18E/F Super Hornet and the Eurofighter Typhoon -- that have already been deployed in combat, Japan opted to purchase 42 F-35 Lightning II jets, built by prime contractor Lockheed Martin, at a cost of $123 million apiece.
The first four aircraft were to be delivered by March 2016.
Delays and cost overruns have plagued the program and earlier this month Japan's Defense Ministry informed the Pentagon about the possibility of canceling the order over concerns that it might not meet the cost.
Other countries proposing to buy the F-35 Lightning II, which is also known as the Joint Strike Fighter Program, include Canada, Britain, Norway, Denmark, Holland, Italy, Turkey and Australia.
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