Chad has suspended all work by the local unit of China National Petroleum Corporation (CNPC) for allegedly violating environmental law, Oil Minister Djerassem Bemadjiele said in a letter seen by AFP on Friday.

The ministerial letter accused CNPC's Chadian subsidiary of systematically carrying out polluting practices banned by both Chadian and international legislation and said that all of the company's exploration and drilling activities were to have been suspended as of May 21.

Chad suspended CNPC activities over similar concerns in August 2013, lifting the ban weeks later, but this time the government in N'Djamena demanded that the Chinese giant pay 1.2 billion dollars (880 million euros) in damages for "a serious violation of regulations on the protection of the environment".

Several dozen research and drilling sites in the poor sub-Saharan nation "have been subjected to dumping harmful to the environment", the letter said, accusing the CNPC of "filling polluted sites without prior treatment" of the waste.

With foreign assistance, Chad began to exploit oil resources in the south of the country in 2003. According to the latest available official figures, the partly desert nation in northern central Africa produced 120,000 barrels per day in 2011.

Chadian authorities and oil workers have already been in conflict with Chinese partners. Last March, trade unions for local oil workers employed by a Chinese consortium organised a brief strike in protest against bad labour conditions and to demand better pay.

The oil income has enabled the regime of President Idriss Deby Itno to modernise the army and to construct new public buildings and improve the road network. However, activists in civil society frequently urge the authorities to use more of the revenue to better living standards for the general population.