A blast has ripped through a warehouse at a US military post near Tokyo, triggering a fire, according to Japanese officials and the Pentagon.

The explosion occurred at a building at the US Army Sagami General Depot in the city of Sagamihara, around 40km southwest of the Japanese capital.

No injuries have been reported

Video footage shot by a local woman showed large sparks shooting out like fireworks lighting up the night sky.

She told public broadcaster NHK that thunderous explosions were heard repeatedly for 10 to 15 minutes.

"Orange sparks were rising quite high. I couldn't see smoke but smelled something like gunpowder," she told NHK.

A spokesman for US Navy Commander Bill Urban said the depot did not store ammunition or radiological material.

Details of building's contents 'unclear'

"The storage building is not designated as a hazardous material storage facility as some initial reports indicated. We are in the process of determining the exact contents of the building," he said.

Japanese and American firefighters initially held off battling the fire to assess the contents of the building, and the blaze died out on its own about six hours after it started shortly before 1.00am local time.

"We coordinated with US fire units, and did not spray water as we waited for information related to what was inside," an official at the Sagamihara fire bureau told AFP.

"We later learned that the warehouse stored various things including oxygen tanks. But exact details were not immediately clear," he added.

About 580 people work at the 200-hectare depot during the day, NHK said.

The cause of the explosion was being investigated by the Americans, the Sagamihara fire official said.

The US, which for 70 years has been the guarantor of Japan's security, has 47,000 service personnel stationed in the country as part of a defence alliance.

Source: Agencies