Japanese Chief Cabinet Secretary Yoshihide Suga called the incident 'extremely regrettable' and said the fire at the warehouse had all but been put out.
The incident comes when Tokyo is at loggerheads with the southern Japanese local government of Okinawa over contentious plans to relocate the US Marine's Futenma air base to a less populous part of the island, and could further stir safety concerns among those who live near US bases.
The local fire department said it received a call just after midnight of an explosion at a US Army depot where it said 'dangerous material' is stored and sent firefighters.
In Washington, US Navy Commander Bill Urban said the blast was at a building at the US Army Sagami General Depot in Sagamihara, about 25 miles southwest of Tokyo. "There are no reports of injury," Urban said.

Inside the storage building, about the size of a large residence, were canisters of compressed gasses such as nitrogen and oxygen, a statement from the US Army said.
A US Army spokesman earlier said the building was not designated as a hazardous material storage facility, and that the depot did not store ammunition or radiological materials.
He also said that no troops lived at the depot, where an estimated 200 personnel work in the daytime. "An incident like this would cause worries among residents living around US military facilities, and is extremely regrettable," Suga told at a conference.
Japan has requested the United States to provide further information on the incident, investigate into causes and prevent a recurrence, Suga said.
A US military helicopter crashed in waters off Okinawa during a training mission earlier this month, injuring seven people and prompting Japan's government to demand similar steps from the United States.


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