More than 40 people were injured in a major explosion at a two-storey izakaya bar in Sapporo last night, Japanese media reports have said, citing local fire department officials.

The police are investigating the cause of the blast, suspected to be a gas explosion, that occurred at about 8.30pm local time (7.30pm in Singapore) at the izakaya Umi Sakura, located near the Hiragishi station on the Sapporo subway line.

No deaths have been reported. Of the 42 reported casualties, 41 suffered light injuries while one has been seriously injured.

The explosion caused the 66-seater eatery to collapse. It also shattered the windows of nearby apartments and food and beverage outlets.

The Hiragishi district is less than 15 minutes by train from the main Sapporo station that serves the capital city of Hokkaido, Japan's northernmost prefecture.

The Hokkaido Police Department and other emergency responders have issued evacuation orders to nearby diners and residents, warning of the possibility of further explosions.

News pictures from public broadcaster NHK at 9pm showed violent flames that firefighters were struggling to contain as emergency responders ferried restaurant employees and diners from the rubble.

Local residents likened the sudden explosion to an earthquake, a thunderbolt and a missile strike, while passers-by continued to report a lingering pungent gas smell.

The Mainichi daily quoted a resident in her 70s as saying: "The restaurant entirely disappeared without a trace, while the roof of the family restaurant next door also collapsed. I'm at a loss for words to describe how I feel about the scale of this catastrophe."

NHK cited the elder sister of a waitress, who has been rushed to hospital, as saying: "She was told to jump off from the second floor, where she had been, and so she did. It seems like she broke her leg, but I am just thankful that she is alive."