No one was injured when a blast ripped through a warehouse at a U.S. military installation in Japan on Monday.

The incident occurred at the Army's Sagami General Depot in the city of Sagamihara, nearly 25 miles southwest of Tokyo,  Agence France-Presse reported. The explosion sent sparks flying and started a fire that burned through the night. Japanese and base firefighters delayed battling the fire as they assessed the materials the damaged building contained.

About 500 Japanese and 300 American troops and civilians work at the 484-acre depot, Lt. Col. Kevin Toner, the chief of public affairs for U.S. Army Japan, told Fox News. The installation sits in the middle of a heavily populated area, and at least one local resident reported smoke entering his home due to the blast.

Meanwhile, officials sought to alleviate fears that the depot may have contained dangerous items, after witnesses said they initially feared a bomb had gone off, Reuters noted. "I thought the American military facility came under a terrorist attack," an unnamed local security guard told Jiji Press news agency, according to AFP.

But "the building that exploded was not a hazardous material storage facility," Toner told Fox. "We are in the process of determining the contents of the building. The depot does not store ammunition or radiological materials."

About six hours after the fire had started in the early morning hours, it died out on its own, an unidentified official at the Sagamihara fire bureau told AFP. "We coordinated with U.S. fire units and did not spray water as we waited for information related to what was inside," he reiterated.

More than 47,000 troops continue to maintain the U.S. military's presence in Japan, the AP noted, and the government in Tokyo urged American authorities to conduct an open inquiry into what led to Monday's blast.

"This sort of incident triggers anxiety among residents living near the U.S. bases, and we urge the (United States) to provide further information, to investigate the cause and to prevent the repetition of such incidents," said spokesman Yoshihide Suga.

Watch the explosion unfold: