A propane tank explosion in a Montreal apartment building has left five people injured, two of them critically.

Paramedics say the two severely injured people had burns on roughly 80 per cent of their bodies but no impact wounds, which are typical in explosions. A firefighter suffered minor injuries.

Firefighters say a preliminary investigation indicates the explosion occurred near a ground-floor staircase.

Roughly 100 firefighters were on scene, as well as police and paramedics.

Police have cordoned off the area and experts are examining the building to determine whether it's stable.

Spokeswoman Lynn Duncan says the investigation has been transferred over to the force's arson squad to determine the cause of the explosion.

"The initial cause would be a propane tank explosion, causing a fire that was quickly controlled by the firemen," she said.

The explosion occurred at around noon at a three-storey, 52-unit rooming house at 1460 Bélanger Street, between Garnier and Fabre streets, on the border of Montreal's Rosemont—La-Petite-Patrie and Villeray boroughs. 

The force of the explosion blew out the building's windows.