WASHINGTON (AFP) - A blast ripped through a United States military post near Tokyo, Japan early on Monday morning, triggering a fire, the Pentagon confirmed on Sunday, noting that no injuries had been reported.
US Navy Commander Bill Urban, a spokesman, said the explosion occurred at a building the Sagami Depot in the city of Sagamihara, some 40km south-west of Tokyo.
"There are no reports of injury," Cmdr Urban said. "These are all the details we have at this time and will provide further information as we receive it."
"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 Pentagon said the cause of the explosion was not immediately known, but that the building did not store ammunition or "radiological materials".
"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," Cmdr Urban said,
The military depot, located next to a local train station, occupies about 200ha.
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.
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.
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.
The footage also showed 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".
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 1am local time.
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.
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.
The Jiji Press news agency quoted the local fire department and other sources as saying that a warehouse was ablaze and that a resident of the area reported hearing more than 10 explosions.
Gas cylinders were reportedly stored in the warehouse.
More than a dozen fire engines and ambulances were at the site but firefighting operations were being hampered by US military restrictions and the possibility of dangerous substances being burned, Jiji Press said, without elaborating.
It added that there was no danger of it spreading since there were no adjacent buildings.
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.