China's environment ministry has suspended approvals of new projects by the country's two biggest oil firms for failing to achieve pollution reduction goals, it said Thursday.

The ministry said it had temporarily halted approval of new refining projects, and renovation or expansion of existing facilities, for China National Petroleum Corp (CNPC) and China Petroleum and Chemical Corp (Sinopec).

CNPC failed to meet the target for reducing chemical oxygen demand -- a measure of the amount of pollution in water -- for last year, the ministry said in a statement.

Separately, it said Sinopec failed to achieve the required cut in its emissions of nitrogen oxide, an air pollutant.

It was not clear how the environment ministry, which is considered weak by many analysts, would enforce the ruling.

The ministry declined to comment further. A Sinopec spokesman said in a statement that the company would "obey" the ministry, while CNPC could not be reached.

Sinopec has previously pledged to invest 22.9 billion yuan ($3.7 billion) on a total of 803 environmental protection projects in the next three years.

China has been trying to tackle air pollution, which has become a key source of social discontent, especially after toxic smog blanketed northern cities earlier this year.

This month, China's top economic planner called for greater efforts in energy-saving and emission reduction, adding assessments of government officials will be partly based on their achievements in the area.

The environmental ministry said it would punish officials in the cities of Beijing and Tianjin, and the neighbouring province of Hebei, for heavy air pollution if they were found to have neglected their duties, the official Xinhua news agency said Thursday.

China has overtaken the United States as the top producer of the carbon emissions blamed for climate change, mainly due to its reliance on coal and heavy industry.