Authorities had evacuated residents within five km (three miles) of the disaster site, it said.

The blast happened on Tuesday morning at the Chuandongbei gas field in Kaixian county of Chongqing municipality. A well had burst, releasing "a high concentration of natural gas and sulfurated hydrogen", the news agency said.

The field belongs to China National Petroleum Corp (CNPC), parent of oil major PetroChina.

A hospital worker in Kaixian said: "We are very busy. There are patients everywhere. We haven't counted the casualties."

Some had been poisoned and others had suffered chemical burns, he said, declining to give other details or his name.

CNPC officials could not be immediately reached for comment.

Poor safety record

China has a notoriously poor work safety record. More than 95,600 people died in work-related accidents in China in the first nine months of the year, including explosions.

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

In August, PetroChina started construction of a $400 million pipeline to pump natural gas from Chongqing to central China.