The explosion occurred at a building at the Sagami Depot in the city of Sagamihara, southwest of Tokyo, a US military spokesman said in an e-mailed statement on Sunday.

"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," said the spokesman, US Navy Commander Bill Urban, from Washington.

The local fire department said the fire had since subsided and that there was no danger of it spreading further, as there were no buildings next to the warehouse where the explosion occurred around midnight.

The cause of the fire was not immediately clear, and the type of material stored in the warehouse was not known, the fire department said, although it was initially said to be "dangerous."

A US army spokesman however later denied that the building stored any hazardous materials.

"The building that exploded 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," Lieutenant Colonel Kevin Toner told Reuters news agency in an e-mail.

He also said that no troops lived at the depot, although around 200 personnel worked there during the day.

Sagamihara is located some 25 miles (40 kilometers) southwest of Tokyo.

tj/bw (Reuters, AFP)