HEFEI, Sept. 16 -- The search continues for 21 missing coal miners after an explosion in east China's Anhui Province 28 days ago trapped the workers in the mine.

An explosion ripped through the mine at about 4 a.m. on Aug. 19 when a total of 39 workers were in mine shafts hundreds of meters underground. Twelve of them managed to escape.

So far rescuers have retrieved the bodies of six workers, but the search for the remaining miners has been hampered by collapsed mine shafts and gas pockets.

Coal deposits at the site of the accident in Huainan City are deep underground, pushing miners to travel to dangerous depths to extract the ore. The Dongfang Mine shaft, where the workers are trapped, descends 500 meters underground.

The provincial coal mine safety inspection bureau revoked the privately-owned Dongfang coal mine's production permit on Tuesday. The operation has an annual production capacity of 90,000 tonnes.

At the time of the incident the mine was operating illegally.

Although the mine was officially licensed, the city government had issued production suspension orders for all coal mines beginning June 30 as part of a flood prevention effort.

The provincial government on Monday urged a thorough overhaul of mines with an annual capacity equal to or lower than 90,000 tonnes, and pledged to provide subsidies for closed mines.

Bai Fafu, a miner who narrowly escaped the Dongfang mine blast, told Xinhua that although the mining is risky, he would seek jobs in other coal mines no matter what, as it was the highest form of income in the local area.

Farmers have surrendered their land to mining firms. The deep-well drilling has damaged the land, making it impossible to be reclaimed after the mineral resources are dug out.