A couple and their adult son are dead after a fire in Ottawa's Britannia neighbourhood early Monday morning.

Emergency crews were called just before 3 a.m. to a three-storey home on Britannia Road between Britannia Park and Mud Lake, according to an Ottawa Fire Services (OFS) news release.

Firefighters were met with intense flames and heavy smoke as they pulled three adults from the second floor of the home, OFS Deputy Chief Paul Hutt said.

A woman in her 80s, a man in his 70s and another man in his 40s were later pronounced dead. All three were members of the same family, Ottawa police said.

A family dog also died in the fire.

Ottawa firefighters, police and the Ontario Fire Marshal's office are investigating the Britannia Road fire. (David Richard/CBC)

Firefighters called for extra help at about 3:15 a.m. and had the flames under control about half an hour later. Neighbours told CBC they were woken up by the sound of a chainsaw around 4 a.m. before noticing their power was out.

The fire is believed to have started in one of the bedrooms, said Sgt. David Christie with the Ottawa police arson unit.

"It's a terrible thing. It's just horrible," said Roxanne Charlebois, who lives in Britannia Bay and described the area as close-knit. She said she often saw the father and son out walking their dog together.

The heritage plaque on 127 Britannia Rd. is visible to the left of the ground floor window. (David Richard/CBC)

The cause of the fire is under investigation, but police said it may take some time to assess the scene because of an unstable floor. The Ontario Fire Marshal's office has been notified.

The house at 127 Britannia Rd. received a historical designation from the City of Ottawa in 2004 and was officially known as the William Murphy House.