Japan's defence minister, Naoki Tanaka, says he has ordered missile defence systems to be prepared in response to the planned launch of a North Korean long-range rocket next month.
Reports say the defence systems will be deployed near the island of Okinawa to shoot down the rocket should it threaten Japanese territory.
North Korea says the rocket will put a satellite into orbit between 12 and 16 April, but the United States and its allies believe the launch is a pretext for a missile test.
Pyongyang said last week it was to mark the 100th birthday of its late "Great Leader" Kim Il-sung with the launch.
The announcement drew widespread criticism that the launch would violate United Nations Security Council resolutions.
The resolutions were imposed after a similar launch in April 2009. Japan is particularly concerned, as the last North Korean rocket was launched over the country.
South Korea and China have also expressed concern over the planned launch.
Last month, Pyongyang agreed to suspend long-range missile tests as part of a deal under which the US would supply 240,000 tonnes of food aid to North Korea.
A US state department spokesperson says it would be "hard to imagine" giving food aid if Pyongyang went ahead with the rocket launch.