The gas well that exploded and killed at least 233 people in southwest China is to remain closed until practical safety measures have been taken to prevent a repeat of the tragedy.

Wang Xianzheng, director of the State Administration of Work Safety and deputy director of the work safety committee of the State Council, who leads the survey team of the State Council organized after the disaster in Chongqing Municipality, made the announcement.

The gas well in Gaoqiao Township of Kaixian County blew up suddenly last Tuesday night, releasing a toxic gas that poisoned more than 9,000 nearby villagers.

More than 20,000 people in the area who had been evacuated began to return home yesterday.

Five villages - all in Gaoqiao Township - were the most severely affected.

"No gas has escaped since the successful sealing of the well on Saturday," said Zeng Shitian, an engineer at the Sichuan Petroleum Administration Bureau.

Rains forecast for the near future are expected to help the toxic substances sink into the ground, according to a local meteorologist.

Thousands more residents returned from outlying areas on Sunday, a day after technicians wearing respirators and protective suits plugged the well with tons of sealant.

A total of 64,000 people fled the remote, mountainous area after the disaster. Many have been sleeping on floors in schools and government buildings in nearby towns.

Emergency crews were searching the area for survivors and more bodies, and scientists have tested water and plants for poison residue.

Survivors described the gas as an acrid black fog that clung to the ground, choking them and burning their eyes and skin.

"According to data analysis of the site, the atmosphere and surface water quality are secure enough for villages to return home," said Zhang Shaozhi, director of the Chongqing Environmental Protection Bureau.

From Sunday morning, a 600-strong team began a massive effort to disinfect the area.

Zhang Hongxin, deputy head of Kaixian County, said vehicles and food had been prepared for evacuees by the county government, and everybody had been given an umbrella.

Environmental experts advised residents to open their windows and doors to ventilate their houses.

Any water that had been preserved prior to the accident should be discarded, they warned. Grain and vegetables, however, are deemed safe to ingest.

Electricity supplies have been secured by the local government. Shops are being encouraged to open to guarantee the supply of daily necessities. Medical personnel are accompanying victims home to guarantee timely treatment.

Relatives of the dead were advised not to return as bodies were still being identified.

By Sunday, 110 bodies from 58 families had been confirmed, said Du Lianjian, director of the civil affairs bureau in the Kaixian County yesterday morning.

Eight teams in charge of body transportation, cleaning, preservation, photography, registration, numbering, confirmation and claims have been organized at the local funeral home, Du said.

Ninety work teams have been formed to offer emotional support for relatives of those who perished.