Iran Press/ Asia: Initial reports on Japanese media suggested at least 40 people were injured, with many rushed to hospital in ambulances.
The cause of the explosion in the Toyohira district of Sapporo is not yet known, according to a Reuters report.
Images on social media initially showed flames rising from the area with debris all around, and later firefighters tackling collapsed buildings.
Police have sealed off the area amid fears of more explosions.
Japanese broadcaster NHK said the area affected had both residential and dining establishments and was about 3km south-east of the city centre.
The Japan Times carried an image of firefighters working at a collapsed building and quoted one eyewitness as saying the explosion sounded like thunder.
Another eyewitness told NHK that the blast had broken the windows of the restaurant he was working in, and that there were "many injured people". 103 / 211 / 209
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