A WOMAN who was grabbed from behind and set alight outside a brothel probably knew her attacker, police say.

Police Acting Superintendent Leanne McCusker said an accelerant was used to ignite the fire which caused burns to 40 per cent of her body.

The 33-year-old victim was set upon as she walked alone down a Chippendale lane way about 9.45pm on Thursday.

Police say the man in a dark hooded top ran away as the woman's head, face, chest and arms became engulfed in flames.

Acting Inspector Sam Crisafulli told ABC radio today that bystanders saw the attack and rushed to her aid.

"They heard the noise and came to help.

"They have taken her inside and attempted to extinguish the flames with water."

Brothel workers ran from the building to help the woman and doused her in water.

She is in a serious but stable condition at Concord Hospital with severe burns and is not yet well enough to be interviewed by police.

Police have not confirmed what the woman was doing near the brothel, but do not believe the attack to be random.

"Investigations do indicate that it was a targeted attack," Act Supt McCusker said.

Officers are attempting to locate security vision from the surrounding area.

"Police from Redfern local area command are seeking assistance and asking for witnesses to come forward who may have seen a male person running from the scene," Act Supt McCusker said.

The man is of medium build and was wearing black pants and a long sleeved dark coloured jumper.

Anyone with information about the incident is urged to contact Redfern Local Area Command or Crime Stoppers on 1 800 333 000.

Originally published as Woman screamed while set alight