Japan's defense ministry is moving toward selecting Lockheed Martin Corp's F-35 jet as the mainstay of its next-generation fighter force, Kyodo news agency reported on Monday, citing ministry sources.

The ministry, which is thinking of buying about 40 F-35s, plans to submit a request for their purchase in the fiscal 2011/12 budget, Kyodo said.

It added, however, that the ministry may postpone making the request for one year, due to a view in the government that the purchase should only be made when full details of the plane's capabilities are available.

Lockheed is developing three models of the radar-evading F-35 to replace at least 13 types of aircraft, initially for 11 nations.

Kyodo said each F-35 was estimated to cost about 9 billion yen ($100 million).

Japan is looking to replace its current fleet of aging F-4 jet fighters, whose design dates back to the 1960s, because they have become increasingly difficult to maintain.

Lockheed said last week that it was making progress on the F-35 fighter, the costliest U.S. weapons program, despite rising costs and delays.

Japan has also been following moves in the U.S. Congress aimed at extending production of Lockheed's radar-evading F-22 Raptor fighter, widely considered the most advanced fighter plane in use today.

Foreign sales of the F-22 were banned by a 1998 law aimed at protecting the stealth technology and other high-tech features. A Senate panel has urged the Air Force to start developing an export model of the F-22.

Apart from Japan, Israel and Australia have also shown an interest in buying the F-22.

(Reporting by Miho Yoshikawa; Editing by Alex Richardson)