A blast ripped through a US military post near Tokyo early Monday, triggering a fire, the Pentagon confirmed, noting that no injuries had been reported.
US Navy Commander Bill Urban, a spokesman, said 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.
"There are no reports of injury, and base firefighters and first responders are currently fighting the resulting fire to prevent its spread to nearby buildings," Urban said.
Japanese firefighters said they were waiting for information from the US side on what was stored in the warehouse before tackling the blaze.
"We are coordinating with US fire units and we are waiting for people who know exactly what is inside to tell us what's inside," a duty officer at the Sagahimara fire bureau told AFP.
"We have not poured water (on the warehouse) because it could make the situation worse, depending on what is inside," he said.
Commander Urban later 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.
The cause of the explosion was being investigated.
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 national 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 open flames rising outside the building, with a smaller orange blaze seen inside though its burnt roof.
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 or toxic fumes, he added.
"We have been in touch with the US side and we are now planning to take action when we have daylight," he said later, with the fire apparently weakening.
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.
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