Several Japanese media outlets received a statement from a suspected Japanese ultra-leftist extremist group claiming responsibility for two small explosions outside a U.S. military base south of Tokyo, police said Saturday.

The Revolutionary Army group called the launch "an attack to sabotage" U.S. military realignment in Japan and Vice President Dick Cheney's planned visit to Japan next week, Kyodo News Agency reported.

Police were investigating Monday's attack on Camp Zama after finding a pair of metal tubes and fragments at a nearby park. There were no reports of injury or damage, and nobody has been arrested. Camp Zama is part of the sweeping U.S. military restructuring.

Kanagawa prefectural police spokesman Hiroyoshi Ichikawa said he could not confirm if police also received the letter.

Japanese leftist radicals have in the past launched similar attacks on government facilities and the U.S. military to oppose an airport facility expansion, bilateral security and other issues.

In 2002, two explosions were heard outside Camp Zama, and Japanese police blamed leftist radicals for the attack that caused no injuries.

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