An explosion rocked a warehouse at a US military base in Sagamihara near the Japanese capital of Tokyo on Monday, but US forces said the building was not used for storing hazardous materials and there are no indications of injuries.
Japanese Chief Cabinet Secretary Yoshihide Suga called the incident "extremely regrettable" and said the fire at the warehouse had nearly been put out.
The incident comes as Tokyo is at loggerheads with the local government of the southern Japanese prefecture of Okinawa over contentious plans to relocate the US Marines' 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 regarding an explosion at a US Army depot, where it said "dangerous material" is stored, and sent firefighters.
The cause of the fire was not immediately clear, the fire department said.
In Washington, US Navy Commander Bill Urban said the blast was at a building of the US Army Sagami General Depot in Sagamihara, about 40 km southwest of Tokyo.
"There are no reports of injury," Urban said in an emailed statement.
Inside the storage building, which was about the size of a large residence, there 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 a regular news conference.
Japan has requested the United States provide further information on the incident, investigate into its 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 again demand more caution from the US forces stationed on the island.