Image caption The incident occurred outside the base late on Thursday night

Two "improvised launch devices" were fired near a US military base in Japan late on Thursday, officials said.

No-one was injured in the incident, which took place outside Yokota air base near the capital, Tokyo.

Bangs were reported by local residents and police found steel pipes, wires and batteries at the scene, police said.

It is not clear who was responsible. Japan hosts tens of thousands of US troops under a long-standing security alliance. Yokota is a major US base.

"We can confirm there was an improvised launch device outside Yokota over the evening," Yokota air base said in a statement.

"There were no injuries and thus far we have found no damage or impact points here on base."

Japan's public broadcaster NHK reported that the pipes were stuck in a field about 300 metres from the base, positioned towards it.

The authorities suspected a left-wing radical group could be behind the incident, NHK said.

The incident comes amid increased focus on security issues in East Asia, amid severe tension between Japan and China over disputed islands.