updated 2/12/2007 12:00:06 PM ET 2007-02-12T17:00:06

A small explosion occurred outside a U.S. Army base south of Tokyo late Monday, police and military officials said. A Japanese news report said police suspected an attempted attack on the base.

The Army was investigating the blast, said an official at Camp Zama who spoke on condition of anonymity, adding that there were no reports of injuries or damage.

“A small explosion was heard in the vicinity of the base,” said Maj. David Smith, a Pentagon spokesman. “It did not occur on the base.”

Kyodo News agency reported that police had found a “launch pad” near the base and suspected an attempted guerrilla attack. The Army could not immediately confirm what had caused the explosion.

A Kanagawa Prefecture police official said Zama residents reported hearing the explosion around 11 p.m. and police were investigating.

In 2002, two blasts were heard outside Camp Zama, and Japanese police found a metal projectile and a crude mortar made from a metal pipe nearby. Investigators blamed radical guerrillas for the explosions, which caused no injuries.

Leftist extremists in Japan have used projectile launchers against targets related to the U.S. military or on sites connected to the royal family. The attacks are usually more symbolic than dangerous, and injuries or significant damage are rare.

The United States has some 50,000 troops based in Japan under a security treaty. Residents complain of crime, pollution and noise connected to the bases, but large-scale protests of the military presence are largely confined to the southern island of Okinawa.

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