"Unknown conditions had arisen from deep down the well," Xinhua quoted a source with the accident response headquarters as saying.

The Luojia No. 2 natural gas well in Gaoqiao town, Kaixian county, is one of largest in the country, and the gas pressures and sulphur content in it are among the highest, Xinhua said.

The well is owned by a subsidiary of China's largest oil firm, China National Petroleum Corporation. It is in the same area where a blast at another CNPC company killed 243 people in December 2003.

With the leak still not fixed, 15,000 people who live close to the well and were evacuated following Saturday's blast are still unable to return home, Xinhua said.

Earlier reports had said 11,500 people had been evacuated and no explanation was given as to why the number had suddenly jumped to 15,000.

Around 9,000 residents have been housed in 12 specially built centers, while 5,000 others have found alternative accommodation, mostly with relatives, Xinhua said.

Local officials are struggling to cope and faced a "dire" need of supplies with just 790 quilts in stock, it said. The county government has launched a campaign to find additional supplies and collected 8,100 articles of clothing.

Unsafe levels of hydrogen sulphide, which causes burning skin and breathing difficulties, had been detected within one kilometer (half a mile) of the leaking well, Xinhua said.

No casualties have been reported although the local government has warned people not to drink water from the local river.

Another accident at a chemical factory in the eastern province of Zhejiang last week led to 51 people being hospitalized, a government official said on Wednesday.

In yet another accident, several thousand residents in the southwest province of Sichuan were evacuated for a few hours on Tuesday when chlorine gas fumes spewed from a pharmaceutical factory, the Beijing Youth Daily said.

The incident was blamed on old electrical wires which sparked a chemical reaction when they came into contact with rainwater, the paper said.

Industrial accidents occur regularly in China, with companies often accused of neglecting safety, equipment and environmental standards in pursuit of profits.

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