Two adults and two children have been killed in a house fire east of Toronto that also sent three other people to hospital, fire officials said Monday.

Fire Chief Derrick Clark said a man, a woman and two children -- a boy and a girl -- died in the morning blaze in Oshawa, Ont. The condition of the three people taken to hospital was not known, he said.

"This is a tragic event," Clark told a news conference. "I want to offer my sincere condolences to the families, all the families involved in this."

Firefighters faced heavy smoke and flames when they arrived at the scene a little after 8 a.m., and had to fight the flames from the outside before attempting to rescue those inside.

"They knocked the fire down from the outside, knowing that there was reports of people trapped, and made entry and made their way up to the upper levels of the house," Clark said.

At one point, fire crews had to pull back as conditions worsened before regrouping and resuming their search and rescue efforts, Clark said.

The residence had been divided into a number of apartments, he said.

"From our initial reports we have two units in this house, but possibly more," he said.

Large rectangular holes could be seen in the roof of the brown brick three-storey house hours after the height of the blaze. Three front windows were completely knocked out and black scorched interior walls could be seen from outside.

Emergency tape cordoned off approximately a one-block area around the house and one fire truck remained parked outside as firefighters examined the residence.

The cause of the fire wasn't yet known, but damage was most extensive at the rear of the building, Clark said.

Whether the home had working smoke detectors would be part of the investigation into the blaze, he said, noting crews faced difficult conditions.

"This fire was at the height the snow storm this morning ... very difficult conditions, both for crews driving en route here as well as fighting this fire," he said.

A woman who works at a nearby convenience store, CW FoodMart, said when she got to work, the home two doors down was on fire with smoke billowing out and flames shooting through the top floor.

She said residents of the home often shopped at the store, including a family with two children and their mother.

"It's very sad, very sad," said Elena, who didn't want to provide her last name.

Oshawa Mayor John Henry said it's always upsetting when there's a loss of life.

"To the families, we pass on our deepest sympathies," he said.
Ontario's Office of the Fire Marshal will be working with local officials to investigate the blaze.