A Nipawin house fire Wednesday night killed four people and injured three others. The cause isn't known. ((David Shield/CBC))

Four people died and three were injured in a fire in Nipawin, Sask., on Wednesday night.

A six-year-old boy and his twin sister died. A woman, 21, and a man whose age is unknown also perished.

A woman, 30, was rescued from the blaze and airlifted from the community, about 140 kilometres east of Prince Albert, to a hospital in Saskatoon. She's listed in serious condition.

Two rescuers were taken to hospital for treatment as well.

The fire was reported Wednesday night at 11:40 p.m.

"I looked out the window and I noticed flames in the sky," neighbour Gelores Berry said. "Then there was white billowing smoke going straight up. I mean, it was enormous."

When fire crews arrived, the small, single-storey home was engulfed in flames.

"There was so much smoke and steam, it was impossible from my angle to tell where the flames were coming from," emergency medical technician Mark Ollinger said.

On Thursday morning, the interior of the house appeared to have been reduced to ashes. Part of the roof had caved in and the windows were broken.

The RCMP and the fire commissioner's office are investigating. There's been no word yet on what might have caused the blaze.

The names of the victims haven't been released, pending notification of family members.