Japan kept security tight across the country Saturday, as its largest contingent of troops to date left on a humanitarian mission to Iraq.

About 140 ground troops attended a departing ceremony before setting off from Chitose airport on the northern island of Hokkaido. They will join about 100 troops constructing a base near the town of Samawa in southern Iraq.

The deployment has stirred criticism at home, with opponents calling it a violation of Japan's pacifist constitution. The Japanese government says the troops are being sent to Iraq to help its people rebuild the country.

The troops left a day after Japan tightened security at airports, nuclear plants and other installations to guard against possible terrorist attacks.

An official speaking on condition of anonymity Friday, refused to say whether the government had new information about a possible strike. But he said 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 that caused no injuries.

Japanese news reports said a leftist group claimed responsibility, saying it opposes deployment of Japanese troops to Iraq.

Another 500 soldiers are to be deployed by late March, backed up by 400 naval and air force personnel in the area.