Two neighbours tried to save the man trapped inside a burning unit in Preston, in Melbourne's north.

Murray Calgaret and Dean Kickett were alerted to the fire by a woman screaming hysterically outside the Preston home.

"The woman was saying 'there's a man in there'. She was saying 'help, somebody help, my house is on fire'," Mr Calgaret told AAP.

The two men then tried to get inside the unit by breaking down the door and smashing a window with a chair, but the intense heat and smoke drove them back.

"We got as far as four steps into the house," Mr Calgaret said.

"I couldn't see, other than that it was pure black smoke. Once I opened the door, it actually went whoosh.

"It was shocking. I am so disturbed. I could have saved somebody's life."

The Melbourne Fire Brigade is investigating what caused the fire that engulfed the public housing unit in Tyler Street, with an arson chemist due to examine the scene.

However, Mr Calgaret said the distressed woman had talked about a gas cylinder and a petrol can.

MFB senior station officer Luke Klein said the fire began just after 3pm on Friday, and was contained within 10 minutes by about 20 firefighters.

He said the Department of Health and Human Services and the MFB were working to relocate the occupants from the neighbouring units.

with AAP