The death toll from a gas leak at a steel plant in north China has risen to 21, after executives admitted they had covered up the true scale of the accident, state media said Friday.

When the gas leak occurred at the Puyang Iron and Steel Co. in Hebei province on Monday, officials initially said that only seven workers were killed, Xinhua news agency said, citing local authorities.

But company executives confessed Thursday that it had covered up the death toll during questioning by a joint investigative group made up of police and work safety officials, the report said.

The deputy manager of the plant was subsequently arrested and two other factory officials placed under house arrest for the alleged cover-up, it said.

The gas leak occurred as workers were assembling a furnace, it added.

China's work safety record is notoriously bad, with thousands of people dying every year in mines, factories and on construction sites, according to official tallies.

In northwest Gansu province, five people were killed and one remained missing after a huge explosion Thursday at a chemical plant belonging to state-run China National Petroleum Corporation (CNPC), Xinhua said.

The explosion and fire at the plant were seen as far away as 20 kilometres (12 miles), the report said.

An initial investigation indicated a gas leak was to blame.

The blast came after a diesel pipeline run by CNPC, China's largest oil refiner, ruptured late last month, seriously polluting two tributaries of the Yellow River and threatening drinking water supplies for millions, reports said.

Also on Thursday, more than 150 workers at an electrical factory in southern Guangdong province tested positive for suspected mercury poisoning stemming from lax work safety regulations, reports said.