A blast ripped through a warehouse at a US military post near Tokyo early Monday, sending sparks shooting into the sky and triggering a blaze that burned through the night, but there were no reports of injuries.

Japanese and US base firefighters held off battling the fire as the contents of the building were assessed, and the blaze died out on its own about six hours after it started shortly before 1:00 am (1600 GMT) with firefighters standing by to contain the blaze.

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

"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, adding that more than dozen fire engines were dispatched to the scene.

Several hours later, a huge blaze broke out at a steel pipe plant near Tokyo's Haneda airport - in the same prefecture - but local police declined to speculate on whether there was any link between the two incidents.

"We do not know any details at this point," a police spokesman told AFP on the question of any connection.

The military depot, located next to a local train station, occupies about 200 hectares (two square kilometres).

Nearly 600 people work at the facility which stores supplies and acts as a repair centre for military vehicles.

Dramatic video footage showed large sparks -- possibly metal canisters -- shooting out like fireworks from a huge fire on the building's roof, lighting up the night sky.

A woman who saw the aftermath of the blast told public broadcaster NHK that she heard repeated thunderous explosions for 10 to 15 minutes.

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

- Under investigation -

The blast rattled the windows at nearby buildings and those on the scene said they feared it could be a bomb.

"I thought the American military facility came under a terrorist attack," a local security guard told Jiji Press news agency.

The Pentagon said the cause of the explosion was not immediately known, but that the building did not store ammunition or "radiological materials". Nearby buildings were not damaged.

"The storage building is not designated as a hazardous material storage facility as some initial reports indicated," US Navy Commander Bill Urban, a spokesman, said in a statement.

"Inside the building that exploded were canisters of compressed gasses: nitrogen, oxygen, freon, and air.

"The cause of the explosion... is under investigation. There are no indications of injuries."

Aerial footage showed the one-storey building's roof had partially collapsed, while the US military said windows and doors were damaged on the building, which is made of concrete and "about the size of a large residence".

The dramatic scene comes after a series of explosions at a hazardous goods facility this month rocked the northern Chinese port city of Tianjin, killing at least 123 people.

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.