The White House says Japan's move to a high state of terror alert is related to an impending Japanese troop deployment in Iraq.
Tokyo is sending a thousand air, sea and ground troops to Iraq -- its largest overseas deployment since World War II. The ground troops will take part in school and road-building in southern Iraq.
White House spokesman Scott McClellan says the Japanese have bolstered police readiness as a precaution.
Heavily armed police have been seen deploying at Japan's airports, nuclear plants and government offices.
It's the first time Japan has gone on such heightened alert since the US led invasion of Iraq.