NANYANG TOWNSHIP, CHINA (BNO NEWS) — Fifteen workers were killed on early Thursday morning when a tramcar derailed and rolled down a tunnel at a coal mine in central China, government officials said. Three others were injured.

The accident happened at around 12:30 a.m. local time when a tramcar carrying a number of miners derailed at a coal mine in Nanyang township of Leiyang city, which is located in central China’s Hunan Province. Few details were immediately released.

According to the State Administration of Work Safety, six of the eight mine cars broke loose and derailed before rolling down a tunnel, killing 15 of the workers on board. Three others survived the accident but were injured, the Administration said, without disclosing their conditions.

According to the state-run Xinhua news agency, an initial investigation found that the miners had violated safety rules by riding in the mine cars, which are designed to transport coal only. Police have arrested the managers of the mine, the report said.

Safety conditions at mines in China have significantly improved in recent years but they remain among the world’s most dangerous with 1,083 fatalities in the first seven months of 2011 alone. There were 2,433 fatalities in 2010 and 2,631 in 2009.

China in recent years shut down scores of small mines to improve safety and efficiency in the mining industry. The country has also ordered all mines to build emergency shelter systems by June 2013 which are to be equipped with machines to produce oxygen and air conditioning, protective walls and airtight doors to protect workers against toxic gases and other hazardous factors.

The first manned test of such a permanent underground chamber was carried out in August 2011 when around 100 people – including managers, engineers, miners, medical staff, and the chamber’s developers – took part in a 48-hour test at a mine owned by the China National Coal Group in the city of Shuozhou in northern China’s Shanxi Province.

One of the worst mining accidents in China in recent years happened in November 2009 when 104 workers were killed after several explosions at a coal mine in Heilongjiang province.