An explosion has torn through a building in downtown Tokyo, scattering debris across a busy intersection and sending smoke into the air, but the fire was soon contained and only four people suffered light injuries.

Video footage aired by public broadcaster NHK on Monday showed flames through the windows of the shattered second floor of the building, located in the Shinbashi area of the Japanese capital.

The cause of the blast was not immediately clear but witnesses said they had smelled gas prior to hearing the explosion.

“I’d just got to work and was starting preparations when there was a really loud explosion,” said Shinobu Nakagawa, a 26-year-old restaurant worker, who works on the first floor of the building.

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He said he grabbed his phone and rushed outside.

“Furniture and shards of glass were scattered everywhere, and when I looked up, smoke was surging upwards.”

The area where the explosion occurred is near a railway station and packed with bars and restaurants popular with office workers.

-AAP