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Japan has chosen the US-made F-35 stealth fighter jet to replace its aging F-4s as the mainstay of its air defence fleet.

Tokyo announced it would buy a total of 42 of the Joint Strike Fighters from defence giant Lockheed Martin, in a deal worth some $8bn (£5bn).

The jet was chosen over the Eurofighter Typhoon and Boeing F/A-18 Super Hornet.

The announcement comes amid regional uncertainty following the death of North Korean leader Kim Jong-il.

There is also growing concern in Japan about China's increasing military might. In recent months, Japan has said it is bolstering its coastal defences after warning of an increase in Chinese naval activity in the region.

Japanese Defence Minister Yasuo Ichikawa said the security environment in which the fighters would operate was undergoing a transformation, but that the F-35 had "capabilities that can firmly respond to the changes".

The decision had been widely expected, given the close ties between Tokyo and Washington.

The US is Japan's main security ally and the two countries regularly conduct joint military drills in the region.

The move will be a major boost to Lockheed Martin, whose F-35 programme has been criticised in the US for its high cost and production delays. Analysts say it is likely to trigger more orders, including from South Korea, which is also seeking to replace its fighter fleet.

The exact details of the contract, including numbers and timings, are yet to be determined, but in a boost to the domestic economy, officials in Tokyo said Japanese firms would be involved in production of the jets.

The decision will, however, be a disappointment for European manufacturers behind the Eurofighter Typhoon, including British defence company BAE Systems, and to US-based Boeing, producer of the Super Hornet.

The Asia-Pacific region has been rattled by the news of long-time North Korean leader Kim Jong-il's death, amid concerns about what direction the isolated state could take under its new leader, Kim Jong-un.