TOKYO • Japanese police were investigating yesterday the cause of a powerful blast that ripped through a restaurant in the country's north overnight, injuring 42 people and damaging surrounding buildings.

The explosion in the city of Sapporo, in Hokkaido, at about 8.30pm on Sunday started fires and caused the partial collapse of some neighbouring buildings, forcing dozens of residents into shelters, according to officials.

The cause of the blast was still under investigation, police said, but there were reports that it may have been a gas leak.

Some reports said fumes were leaking from spray cans that had been collected for disposal at a real estate company in the building.

But investigators were also looking into at least nine propane tanks installed in the building for the various businesses located inside.

Numerous customers at the restaurant on the building's first floor said they smelled gas at the time of the explosion, Kyodo news agency reported.

The blast produced large flames and sent plumes of smoke rising into the night air, with witnesses describing a terrifying boom.

The two-storey wooden building, which also housed a clinic, was badly damaged, a Sapporo fire department official said.

Police said there were no fatalities but 42 people had been injured. Among those hurt were several children, as well as a man who suffered burns and a woman who jumped from the building to escape, breaking her leg in the process.

"There was an enormous sound, 'bang', then when I looked up at the sky it was filled with plumes of smoke," an elderly woman said in footage on public broadcaster NHK.

"The ceiling fell, then the entire second floor collapsed and we were all stuck. We managed to escape after everyone kicked through the wall," a person who was in the restaurant at the time told Kyodo.

The fire from the blast spread to neighbouring buildings, and debris blasted out by the explosion shattered the windows of nearby apartments and restaurants, according to local reports.

"We are investigating details about the damage together with police at the scene," the fire department official said.

The city government opened a shelter to house dozens of people whose homes were damaged.

The explosion also caused a temporary blackout at 250 buildings.

The three tenants in the building were poorly prepared for possible fires, Sapporo fire department official Takashi Shida told news agency Agence France-Presse.

"We instructed them to improve these points during an on-site inspection in October but they had not made progress," he said.

Many small and medium-sized older buildings in Japan are built partly or entirely from wood and are vulnerable to fires.

In February, 11 people were killed in Sapporo after a fire broke out at a home for elderly people with financial difficulties.

AGENCE FRANCE-PRESSE