Two explosions were heard outside a US naval air station near Tokyo at around 10:30 pm (1330 GMT) Thursday night, police said.

Upon investigating, police found two metal pipes, apparently used as projectile launchers, in a thicket on the grounds of an elementary school, set in the earth and pointed at the Atsugi army base some 650 metres (yards) away.

"It appears to be a guerrilla incident," said Kanagawa prefecture police spokesman Tsuneo Kosuge. "The pipes were aimed at the US base but we haven't found the projectiles, which we are now looking for."

There were no indications of damage, he said.

Reports said police suspected the attacks were part of protests against the war on Iraq, but Kosuge would not speculate on the motive.

US bases have been targeted a number of times over the years by Japanese extremists, but without serious consequences.

In March two metal projectiles were fired at the US Yokota base in western Tokyo from steel-pipe launchers set off by a time device. No injuries or damage were reported.

In November, police said they suspected extremists had used pipe launchers to fire steel balls at the US army base Camp Zama southwest of Tokyo. No one was injured in the attack.

SPACE.WIRE