Relatives of miners who were killed in a gas explosion cry outside the entrance of Xinxing coal mine in Hegang in Heilongjiang province on Nov. 23. (Andy Wong/Associated Press)

The coal mine in northern China where 104 people were killed in a gas explosion on Saturday had too many workers underground, a government official said Monday.

The head of China's State Administration of Work Safety told the official Xinhua News Agency that the mine's management failed to evacuate the facility promptly, but his deputy went further.

The mine had too many platforms and workers inside in an effort to increase output, Zhao Tiechui told Xinhua. He called the structure "far too complicated for its current ventilation system to work effectively."

He did not specify what the mine's capacity was.

The candid comments were a possible attempt to keep relatives' frustration over the incident from turning into one of the many protests that the government sees as threatening stability.

Grieving relatives were shocked that the such a blast could occur at one of China's state-run mines, which the government has promoted as being safer than smaller, privately run concerns.

Xinhua said four more miners are missing and feared dead. The search for the missing miners is continuing.

Roughly 500 miners were working underground on Saturday at the Xinxing coal mine when a gas explosion occurred.

The death toll is the highest for China's mining industry in two years.

The mine belongs to Heilongjiang Longmei Mining Holding Group.

Grieving relatives

On Monday at the gates of the company's offices, family members demanded answers from the company. They were later escorted by police and security into an office.

"Why haven't they told us anything?" said Liu Shujiu, whose 38-year-old husband Zhang Shulai was among the victims. She broke down in tears as she sat in a chair.

"We had to hear from others at the mine," she said.

A mine official, who would not give his name out of fear of government reprisal, admitted that the delay in giving families news about the accident was a mistake.

"In this, we were wrong," the official said.

An initial investigation found that mine management didn't evacuate workers promptly after very high gas density was detected in the pit, Luo Lin, the head of China's state administration of work safety, told Xinhua.

The Xinxing mine's director, deputy director and chief engineer were reported to have been fired following the explosion.

The latest blast highlights the safety problems in China's mining industry. Through the first six months of the year, 1,175 people died in mine accidents, although that was down more than 18 per cent from same period last year.

The Chinese government has shut down about 1,000 smaller mines, many of them illegal. That move has helped cut down on fatalities.

With files from The Associated Press