China has experienced "the worst air pollution this year" with heavy smog blanketing north, east and central China since Nov 30, the Ministry of Ecology and Environment said on Monday.

Affected areas include Beijing and Tianjin municipalities, North China's Hebei province, and Central China's Hunan and Hubei provinces, as well as the Yangtze River Delta area, the ministry said.

The smog came amid high humidity and poor air diffusion in the regions, it said.

Red alert, the highest level in a four-tier warning system, which indicates the Air Quality Index will exceed 300 for more than three days, has been issued for six cities in those areas.

The red alert means factories considered to be heavy polluters will have to shut down and there will be restrictions on motorists.

It also said air conditions are expected to gradually improve as the smog will weaken from Tuesday to Friday as a result of a cold front.