An armed police officer on Tuesday looks out the window of a building burned in a fire that killed nine in Quzhou, Zhejiang province, on Monday night. [Photo/Xinhua]

SHANGHAI - Initial investigations of a fire that killed nine and injured two in East China's Zhejiang province on Monday have indicated arson, local police said on Tuesday.

Police said they have detected traces of combustion promoters in the smoldering remains but did not elaborate.

The fire started at 8:15 pm on Monday in an eight-story residential tower in downtown Quzhou, 230 km southwest of the provincial capital of Hangzhou.

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Witnesses said they heard a loud bang and believed a gas cylinder had exploded.

Local media reported the fire was extinguished in half an hour by more than 50 firefighters in five fire engines.

The nine casualties comprise eight men and one woman. Seven are locals, while the others are from Fujian province, local police said.

The injured are in stable condition at a nearby hospital.

Quzhou's publicity office head Xu Yongchang said the fire broke out in a third-floor apartment where several burnt mahjong tables were later found.

But it is still too early to say people were gambling in the room, he added.

A resident surnamed Zhang rented the apartment, police said. It is not clear if Zhang was among the dead.

"It's impossible for a chess-and-card room located in an apartment building to get a business license," Shanghai Delson Law Firm property lawyer Chang Jinquan pointed out.

"An apartment with more than one mahjong table is very likely an illegal gambling room, as a family typically doesn't need that many."

Underground gambling is constantly found in the country's apartment buildings.

These rooms pose high fire risks. They are often filled with smoking gamers, and have poor ventilation and inadequate firefighting equipment.

The latest fire accident comes a month after a blaze in a 28-story residential building in downtown Shanghai killed 58 and wounded 71, shocking the nation.

Following the Shanghai inferno, which started when unlicensed workers' welding sparks ignited nylon construction netting and bamboo scaffolding, the State Council issued a circular urging the "resolute prevention and curbing of major fire accidents".

The document's measures included tighter inspections and widespread public education.

The Ministry of Housing and Urban-Rural Development had also ordered "thorough inspections" of fire hazards in high-rises that are under construction or renovation.