Officials in the north Chinese province at the heart of a toxic milk scandal hid a coal mine explosion in July that killed more than 30 miners, state media reported.

The People's Daily said on Wednesday that 22 officials are being investigated by prosecutors and one was arrested, accused of covering up the mine explosion in Hebei province on July 14, weeks before Olympic Games opened in Beijing on Aug 8.

The blast in the unlicensed Lijiawa mine in Yu County, also pronounced Wei County, was set off by explosives illegally stored down the mine shaft. Relatives of the dead were kept quiet through payments and threats, the Xinhua news agency said.

"An investigation has shown that after the explosion, the mine owners and county and township governments colluded to conceal the accident," the acting governor of Hebei, Hu Chunhua, told officials on Tuesday, according to Xinhua.

"This is a shocking case."

The report did not explain why the deaths took so long to come to light. Hebei neighbours the national capital, and its officials there were under intense orders to quell threats to "social stability" before and during the Olympics.

Hebei is also home to Sanlu Dairy, maker of the chemical-tainted milk powder that officials have blamed for killing four children and making many thousands sick. Subsequent detection of the chemical melamine in many Chinese dairy products has sparked alarm and product withdrawals in many export markets.

Sanlu and local officials concealed the problem before and during the Beijing Olympics, the central government has said.

After the mine disaster, the corpses were taken away and kept hidden in surrounding areas, where the grieving families were also taken to be paid off, the report said.

Party chief under investigation

"Cash was used to keep them quiet as well as threats and other means, and the miners and their families were not allowed to reveal the facts to the outside," said the report.

Those under investigation include the county's Communist Party chief, a local police chief and work safety and mining officials from nearby Zhangjiakou city, which oversees the county. The mine owner is also under investigation, Xinhua reported.

An official in the Yu County government refused to answer questions about the disaster.

China depends on coal to fuel much of its ballooning energy needs, and the country's shafts are the deadliest of any major mining nation. Safety officials have strived to cut accidents by shutting smaller mines and tightening inspections.

A total of 3,786 coal miners died in gas blasts, flooding and other accidents last year, down 20 per cent from 2006. And officials reported another drop in deaths in the first 8 months of this year.

But deadly mine accidents are still a near daily staple of news reports, and the Hebei blast was not the only recent disaster to involve suggestions of official concealment.

On Monday, the national work safety administration launched an inquiry into a mine tailing dump collapse in northern province of Shanxi on Aug. 1, it reported on its website.

At the time, local authorities there said 11 people died, but a central government probe in September found 41 dead, as well as six severed limbs.

With files from Reuters