BEIJING, Dec 25 (Reuters) Chinese authorities today struggled to cap a leaking gas well in the southwestern province of Sichuan that prompted 12,000 people to flee their homes, state media reported.

The leak began on Thursday at the Qingxi Number 1 Gas Well, operated by Sinopec Corp. <0386.HK>, in Sichuan's Xuanhan county, prompting emergency workeres to set fire to four sites around the well to burn off the gas.

But a first attempt to plug the leak was unsuccessful, the official Xinhua news agency reported.

''About 300 cubic metres of mud was infused into the well in the first capping, but no effect was achieved,'' Xinhua quoted He Shenghou, deputy chief of the emergency headquarters at the site, as saying.

There were still 20-30 metre (65-98 foot) flames around the well but tests concluded there were no toxic chemicals in the air and some residents were being allowed to return home, the report said.

Gas leaks are not uncommon in China.

In March, a leak at a well in the southwest forced the emergency evacuation of 15,000 and in July, toxic chlorine gas leaking from a rusting pipe in the northwestern region of Ningxia sent 164 people to hospital.

Reuters DKA GC1041