The Royal Newfoundland Constabulary is trying to determine the cause of a fire on Blackmarsh Road in St. John's that's left one woman in critical condition.

The fire broke out Thursday morning around 8 a.m.

St. John's Regional Fire Department Supt. Don Byrne said extra resources had to be called on to combat the blaze, given the frigid temperatures and thick smoke.

"The amount of damage is extensive throughout the home. There was heavy fire on arrival and the fire damage is extended right up through to the second floor," Byrne said.

"For a small residence, there was a lot of fire."

According to Byrne, the woman was partially through the front door when firefighters arrived.

He said at one point there were 19 firefighters on the scene.

The RNC said investigators would likely continue their work at the burned-out home throughout the day.

A neighbouring house also received some damage.

With files from Ariana Kelland