No-one was injured in the blast, which happened at around 1 a.m. local time on Monday. Hours later, a huge blaze damaged a steel pipe plant near Tokyo's Haneda airport.
Dramatic video footage showed large sparks shooting out like fireworks from a huge blaze that followed Monday's explosion at the US Army Sagami General Depot in Sagamihara, a city around 40 kilometers (25 miles) southwest of Tokyo.
The blast ripped through a warehouse in the 2 square kilometer (200 hectare) compound at around 1 a.m. local time (1600 UTC on Sunday). No injuries were reported.
Japanese and US military firefighters decided not to battle the fire amid a lack of clarity over the contents of the building. The blaze burned out on its own about six hours later, leaving a massive hole in the roof.
Eyewitnesses described hearing repeated thunderous explosions for almost 15 minutes, amid fears the facility was under a terrorist attack.
Around 600 people work at the depot, which is a repair center for military vehicles.
The Pentagon said the cause of the blast was not immediately known, adding that the building didn't store ammunition or radioactive materials. Compressed gases including nitrogen, oxygen and freon were kept there.
The blaze at the steel pipe plant sent flames thick smoke into the air, just one kilometer from Tokyo's Haneda airport
A few hours later, a huge blaze broke out at a steel pipe plant near Tokyo's Haneda airport.
A fire official said the blaze happened in a 2-meter cooling tower, without giving further details. There were no injuries.
The facility, which spans more than 20,000 square meters (224,000 square feet) operates a pair of manufacturing lines.
The site is owned by a unit of steelmaker Nippon Steel & Sumitomo Metal.
Despite flames and plumes of thick black smoke rising from the plant, no flights were delayed or canceled at Haneda, the world's fourth-busiest airport by passenger throughput.
Police declined to speculate on whether there was a link between the two incidents.
They follow a series of explosions at a hazardous good facility in Tianjin, China on August 12, which claimed at least 123 lives.
Over the weekend, another blast rocked a chemical plant in China's eastern Shandong province, killing one person.
mm/msh (AFP, AP, Reuters)