'Maliciously lit fire': Three people killed, one unaccounted for in suspicious Sydney blaze

Police believe the deadly fire which tore through a Sydney boarding house, killing three people, was deliberately lit.

Three bodies have been found, officers confirmed, after the blaze erupted on Albermarle Street in Newtown about 1am today.

"Serious concerns are held" for a fourth person believed to be inside, police added.

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The bodies have not been formally identified but police believe they are men.

Assistant Commissioner Peter Cotter said police are treating the blaze as suspicious and deliberate.

"It would be fair to say that some type of accelerant hasn't been used," Assistant Commissioner Cotter said.

"We are treating this as a murder.

"We are treating this as a maliciously lit fire."

Assistant Commissioner Cotter said at least 11 men live in the boarding house, aged from their 40s to 80s.

Firefighters have told 9News they are not sure if the building is at risk of collapsing.

Assistant Commissioner Cotter said they have not been able to search the top floor yet due to concerns about the structural integrity of the building.

"We're not sure how many people we're looking for, there's a number of rooms, what I understand upstairs is a hallway with a number of rooms off the left and right," he said.

"We've not been able to access some of those rooms to identify if there are any occupants or tragically anyone perished in those rooms."

One man was forced to jump from the second storey of the boarding house after the fire rapidly took hold of the building, residents told 9News.

"There are bits falling down, there was fire through the house," a resident said.

"I saw an 80-year-old gentleman on the ground who had to jump out of his window so he's broken his ankles, we had to carry him out of there."

Police said eight residents were able to exit the building, with three people taken to hospital including the 80-year-old man, who is in a critical condition and suffered a heart attack when he arrived at the hospital.

Officers also said another resident was forced to "exit from the second storey" due to the intensity of the fire.

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Assistant Commissioner Cotter called for the owner of the boarding house to come forward to assist with inquiries.

"We know who he is. We have reached out to his series of addresses in Western Sydney, and as yet, we have not been able to make contact with him," he said.

"We would like that gentleman to come forward as a matter of priority, not only to talk to us — particularly to talk to us about who was inside — and the tenancies."

Assistant Commissioner Cotter also called for the public to come forward with any information about tenants in the building.