TAIYUAN - Police in North China's Shanxi province have launched an investigation into an illegal mining case that left a coal mine flooded and 6 people trapped early Tuesday morning, local authorities said.

Six people have been trapped underground since a coal mine in Yangquan city was flooded at about 4 am, when 12 people were conducting illegal mining operations, the local government said.

Six of them managed to escape, including one who has been detained, said Liu Xianyun, a spokesman for the office of the city's security committee.

The miners had illegally dug three pits below a small room in a residential courtyard. A local resident told Xinhua he learned that some people used farm vehicles to transport coal from the courtyard on Sunday evening.

Although rescuers have succeeded in lowering the water level in the pit, they said a lack of geographical data has made their work difficult.

Rescue efforts and further investigation into the accident are under way.