UPDATED 8.13AM: A blast has ripped through a US military post near Tokyo, triggering a fire, the Pentagon has confirmed, noting that no injuries have been reported.

US Navy Commander Bill Urban, a spokesman, said the explosion occurred at a building at the Sagami Depot in the city of Sagamihara, some 40km southwest of the Japanese capital just after midnight on Monday.

"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," Urban said.

Japanese firefighters said they were waiting for information from the US side on what was stored in the warehouse before tackling the blaze.

"We are coordinating with US fire units and we are waiting for people who know exactly what is inside to tell us what's inside," a duty officer at the Sagahimara fire bureau told AFP.

"We have not poured water (on the warehouse) because it could make the situation worse, depending on what is inside," he said.

Video footage shot by a local woman showed large sparks shooting out like fireworks from a huge structural fire lighting up the night sky.

She told national broadcaster NHK that thunderous explosions were heard repeatedly for 10 to 15 minutes.

"Orange sparks were rising quite high. I couldn't see smoke but smelled something like gunpowder," she told NHK.

Aerial footage recorded by NHK about an hour after the fire began showed no open flames rising outside the building, with a smaller orange blaze seen inside though its burnt roof.

The municipal fire bureau had dispatched 13 fire engines and other vehicles to the site, the fire official said.

Firefighters had surveyed the area around the warehouse and had not noticed any physical damage or toxic fumes, he added.

The fire department had not received any reports of injuries associated from the fire, he said.