Man fighting for life after fire rips through granny flat

A man has suffered burns to 70 percent of his body after a fire broke out at a property at Gymea in Sydney's south this morning.

Residents woke to screams for help before emergency services were called to a home in San Remo Ave at 1.30am after reports of a granny flat being on fire.

"She was very distressed, there was an explosion. I told her to run and I was banging on the windows to get everyone else out," resident Linda West told 9NEWS.

Jeff Lindsell, 39, was taken to Royal North Shore Hospital in a critical condition after he ran from the building on fire.

"He just ran straight out and basically fell on the ground and I think someone put the garden hose on him," neighbour Michael Bauer recalled.

The man's father reported the blaze after running into the granny flat to rescue his son.

Officers attached to Sutherland Police Area Command have established a crime scene.

The cause of the fire is under investigation and police are treating the blaze as suspicious.

Police are urging anyone with information in relation to this incident to call Crime Stoppers on 1800 333 000.