A blast ripped through a warehouse at a US military post near Tokyo early Monday, according to Japanese officials and the Pentagon, triggering a fire, but no injuries had been reported.

The explosion occurred at a building at the US Army Sagami General Depot in the city of Sagamihara, some 25 miles (40 kilometres) southwest of the Japanese capital.

Dramatic video footage shot by a local woman showed large sparks shooting out like fireworks from a huge structural fire 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.

Aerial footage recorded by NHK about an hour after the fire began showed no flames rising outside the building, with a smaller orange blaze seen inside though its burnt roof.

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 1:00 am (1600 GMT).

Daytime television footage showed part of the warehouse's roof was burnt, and blackened metal.

"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.

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

"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 earlier.

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

The municipal fire bureau had dispatched 13 fire engine and other vehicles to the site, the fire official said.

Firefighters had surveyed the area around the warehouse and had not noticed any physical damage to nearby buildings or toxic fumes, he added.

The fire department had not received any reports of injuries.

Washington, 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.

A constitution imposed by a post-war US occupation force barred pacifist Japan's military from combat except in self-defence.