Seven people were injured and seven others are missing after an apparent explosion tore through an apartment building in the Ontario community of Woodstock.

Police said it's unknown how many -- if any -- of the missing residents were inside the building at the time of the blast, which happened just after 8:30 a.m.

But with so much of the building destroyed, police were not overly optimistic.

"If we don't have any fatalities, I'll be very surprised," Sgt. Marvin Massecar of Woodstock police said late Sunday afternoon.

One person remains at the Woodstock General Hospital with non-life threatening injuries. Another six people have since been released.

Firefighters battled the blaze for hours until it was finally extinguished. The centre of the building, located at 168 Wellington St. South, appears to have collapsed.

One third-floor resident said he woke up on the first floor after the explosion.

Const. McEwen from the Woodstock police said about 30 firefighters and 20 police officers joined efforts to fight the fire and help those left homeless.

People who evacuated the building were placed in buses and taken to the Woodstock community complex. The Salvation Army set out drinks and food for the victims at a nearby Via Rail station.

McEwen said the apartment building, located between Welling and Victoria Streets, had been standing for at least 20 years and was not sparsely inhabited.

Nearby residents were asking how the explosion could have happened. The building does not use natural gas, as it has electric heating.

Anyone wishing further information is asked to call the Red Cross, who are on scene, on 1-866 280-1735.