A blast ripped through a US military post at the Sagami Depot in the city of Sagamihara, some 40 km southwest of Tokyo on Sunday night but no injuries have been reported so far.
US Navy Commander Bill Urban, in confirming the explosion, said: "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 that 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, adding that the cause of the explosion was being investigated.
A US army spokesman Kevin Toner told Reuters that the building where the explosion took place was not a hazardous material storage facility. "We are in the process of determining the contents of the building. The depot does not store ammunition or radiological materials."
He said that no troops lived at the depot which is the workplace of an estimated 200 personnel.
Video footage shot by a local woman showed large sparks shooting out like fireworks from a huge structural fire lighting up the night sky, AFP reports. The woman told Japan's national broadcaster NHK that thunderous explosions were heard repeatedly for 10 to 15 minutes.
"Orange sparks were rising quite high. I couln't see smoke but smelled something like gunpower," the woman told the TV station.
According to AFP, aerial footage recorded by NHK about an hour after the fire started showed no open flames outside the building but a smaller orange blaze was seen inside.
A total of 13 fire engines and other vehicles were sent to the site, according to a municipal fire bureau offical.
"We have been in touch with the US side and we are now planning to take action when we have daylight," he said, adding that no reports of injuries were reported.
Sagami depot handles army supply and logistics
The Sagami depot is home to the 35th Combat Sustainment Support Battalion which handles army supply and logistics including the management of chemicals and ammunition, according to the Guardian.
According to Japanese terrorist expert, Michael Penn who heads the Tokyo-based news agency Shingetsu News said: "It is a deserted area and the question is how the first was started. Something obviously blew it up."
The Mirror quoted Penn as saying that although Japanese firefighters were called to the scene, there were "restrictions put on them", noting that water was not used to douse the fire.
The Telegraph said that the area had previously been the site of an intentional attack, noting that the Wall Street Journal reported three explosions in April in the vicinity of the base. The perpetrator of the first attack is still not known.