Japan has tightened security at airports, nuclear plants, government facilities and other installations to guard against possible terrorist attacks.
An official speaking on condition of anonymity refused to say whether the government had new information about a possible strike. But he did say this was the highest show of security in Japan since the U.S.-led invasion of Iraq last year.
White House spokesman Scott McClellan says Japan has kept the United States apprised of its tightened security measures, which he called "police preparedness."
The heightened security follows two explosions this week near Japan's defense agency in Tokyo. There were no injuries, but police said they found two projectile launchers.
Japanese media reported a leftist group claimed responsibility, saying it is opposed to the deployment of Japanese troops to Iraq.
Japan has sent about 100 troops to southern Iraq to assist in humanitarian and reconstruction efforts. Another 500 are expected by late March, backed up by 400 naval and air force personnel in the area.