At least 21 miners were killed in a gas burst in a colliery in central China's Henan Province Saturday, the rescue headquarters said.

Rescuers had retrieved 20 bodies by 1 p.m. and were searching for 16 miners believed to be trapped underground after a gas burst at about 6 a.m.in the mine in Yuzhou City, a spokesman for the rescue headquarters said.

A total of 276 miners were working underground when the accident happened and 239 escaped.

More than 70 rescuers are working in shifts despite a high density of gas underground, which is hindering the operation.

The 16 miners have been located, but rescuers must clear the coal dust from the shaft first.

Ventilation has been resumed in the pit.

An initial investigation showed that more than 2,500 tonnes of coal dust were in the pit after gas burst, said Du Bo, an engineer with one of the mine's parent companies.

Du said the accident occurred when workers were taking measures to prevent gas bursts.

The mine is owned by Pingyu Coal & Electric Co. Ltd., a company jointly established by four investors, including Zhong Ping Energy Chemical Group and China Power Investment Corp..

Luo Lin, head of the State Administration of Work Safety, and Zhao Tiechui, head of the State Administration of Coal Mine Safety, are on their way to the site.

The same mine reported another gas and coal burst on Aug. 1, 2008, which left 23 miners dead.