TOKYO (REUTERS, AFP) - Police and fire officials on Monday (Dec 17) searched for the cause of a dramatic explosion in the northern Japanese city of Sapporo that collapsed a building and injured 42 people.

Media reports said the Sunday night incident may have involved propane gas tanks in the building where the blast occurred. Sapporo police and fire officials said it was too early to say anything certain about the cause of the explosion.

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 in the night air, with witnesses describing a terrifying boom.

The two-storey wooden building, which also housed a clinic, was seriously damaged, a Sapporo fire department official told AFP. Police said 42 people had been injured, but there were no fatalities.

Among the injured 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 blast happened around 8.30pm (7:30pm Singapore time), and the city government opened a shelter to house dozens of people whose homes were damaged.

The explosion also caused a temporary black-out, with 250 buildings losing electricity, but power was later restored, he said.

The three tenants in the building were poorly prepared for possible fires, Sapporo fire department official Takashi Shida told AFP. "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.