Tokyo is considering deploying both its ballistic missile interceptor warships to the Sea of Japan ahead of a possible test launch of North Korea's longest-range missile, Kyodo news agency reported on Tuesday.
North Korea has racked up tensions in past weeks with what is believed to be preparations for a test flight of its Taepodong-2 missile, capable of striking US soil, in what is seen as a move to put pressure on Washington to review its Pyongyang policies.
North Korea has said it was preparing to launch a satellite and it had the right to do so as a part of a peaceful space programme.
If preparations become more active, Japan will send two naval destroyers with high-tech Aegis radar systems and Standard Missile-3 (SM-3) interceptors to the Sea of Japan, between Japan and the Korean peninsula, Kyodo quoted a defence ministry official as saying.
Japan will be cooperating with U.S. forces, Kyodo quoted the official as saying. The US has also stationed anti-ballistic missile ships in Japan.
A spokesman for Japan's Defence Ministry said nothing had been decided as yet.
Prime Minister Taro Aso indicated earlier this week that Japan should be able to use its missile defence capabilities even if North Korea insists that it is launching a satellite, domestic media reported.
Failure to shoot down a target could undermine faith in Japan's defence system, which was introduced with U.S. help after North Korea fired a missile over the country in 1998, stunning the region.
Apart from SM-3s, which are designed to shoot missiles down in the mid-phase of flight, Japan has a second layer of defence in the form of land-based PAC-3 interceptors, positioned at military bases mostly close to Tokyo.
A missile heading towards the United States from North Korea would likely be travelling too high for Japan's SM-3s to shoot down, officials have said.
Last November, a Japanese warship failed to shoot down a ballistic missile target in a joint test with U.S. forces because of a glitch in the final stage of an interceptor.