BEIJING (AFP) — Investigators probing a bus blaze in southwest China that killed 27 people have issued a public appeal for help in identifying a person suspected of bringing petrol on board the vehicle.

"From what we've gathered, someone brought flammable material -- gasoline -- onto the bus," investigator He Jiansheng said at a press conference late Sunday in Chengdu, the provincial capital of Sichuan province.

"The fire might have been caused by accident or it could have been started on purpose," he said according to transcripts of the press conference posted on the government's website.

He said police were investigating how the packed single-decker bus burst into flames early Friday in an accident that also left 74 people injured.

Two of the injured died Sunday from severe burns, while 18 others remain in critical condition.

"Because so many people were moving around in the bus during the fire, the investigation of the scene has been difficult and so far it has not been possible to identify who carried the gasoline on board," he said.

"We are calling on anyone with knowledge of this incident to come forward and give information to the police," He said.

Most of the victims were trapped inside the vehicle as the doors failed to open and all of the windows on the air-conditioned bus were shut except for those in the rear.

Passengers recalled smelling petrol and seeing a liquid on the floor of the bus before it burst into flames in a huge explosion, the Chongqing Evening News reported Sunday.

"At the time, I could smell the odour of gasoline along the corridor and on the floor there was a clear liquid that looked like water," the paper quoted a passenger identified as Fu Jun as saying.

The mother of another witness who was at the rear of the bus at the time said her son claimed to have seen a man dump a bottle of liquid on the floor and then immediately jump out of a rear window when the bus burst into flames, the paper said.