BEIJING (Reuters) - At least 191 people were killed and hundreds poisoned when a natural gas well in southwestern China burst and released a cloud of toxic fumes, medical workers, officials and state media said on Thursday.

Authorities evacuated 31,000 people, most of them farmers, from the mountainous area after the government ordered everyone cleared from within five kilometers (three miles) of the site of Tuesday's disaster in Kaixian county, in densely populated Chongqing municipality, the official Xinhua news agency said.

Twenty teams were dispatched to villages in the surrounding area to search for survivors and identify casualties. Local officials feared the death toll would rise, Xinhua said.

Children were among hundreds poisoned or suffering chemical burns, hospital workers said.

"We're very busy. There are patients everywhere. We haven't counted the casualties," said another worker at the hospital.

In hospitals, the injured had swollen and sensitive eyes and some had difficulty standing, the private www.sina.com Web site said. The total number of injured was not clear.

Rescue workers carrying oxygen tanks on their backs and wearing masks were seen near the site, one official told Reuters by telephone.

At one Kaixian hospital, more than 200 people were being treated for poisoning, an administrator said by telephone from the area, which is about 1,000 km (620 miles) west of Shanghai.

China has a notoriously poor work safety record. More than 120,000 people died in work-related accidents from January to November this year, the official China Daily said earlier this month.

The cause of the latest disaster, one of the deadliest of this year, was being investigated. An oil industry official in Chongqing said the well had burst while it was being drilled.

The official Xinhua news agency said at least 191 people had been confirmed dead by Thursday evening.

"Chinese leaders called for unreserved efforts in searching and rescuing victims," it said.

RISING TOLL

Xinhua said the gas well burst on Tuesday, spewing a high concentration of natural gas and sulfurated hydrogen 30 meters (100 feet) into the air at the Chuandongbei field, which is owned by China National Petroleum Corp (CNPC), parent of oil major PetroChina.

Evacuees were organized into 10 shelter areas as temperatures hovered around freezing, a county government official said.

Xinhua said sulphurated hydrogen is a toxic gas that can poison or kill people who inhale 760 to 1,000 milligrams per cubic meter.

It was also unclear why the reported death toll had shot up two days after the accident. Earlier on Thursday, Xinhua reported that 133 had died but soon raised that to 163 and then 191.

An industry source said top CNPC officials said they did not know the exact number of casualties as of Wednesday morning before going to the scene.

"They were still scrambling around, trying to figure out what happened," he said.

State radio said an earlier burst had been brought under control on Wednesday.

Xinhua said 260 cubic meters (9,180 cu ft) of mud, to be pumped into the well to plug it up on Friday morning, and 13 special rescue vehicles were en route to the scene.

The central government sent a team to coordinate the rescue.

Chongqing and the neighboring province of Sichuan are among China's major natural gas producing areas.