The Ministry of Environmental Protection's online monitoring system detected that two coking plants affiliated with Shanxi Coking Group and Shanxi Sanwei respectively, have discharged excessive air pollutants since the beginning of the year.

Following heavy air pollution alerts in 14 cities, including Shijiazhuang in Hebei, Linfen in Shanxi, Heze in Shandong and Hebi in Henan, the ministry sent inspection groups to the cities, toughening scrutiny on polluters.

Inspectors found that six production lines of Shanxi Coking Group failed to install or operate desulfurisation and denitration facilities.

The ministry has ordered the provincial environmental protection authorities to investigate those enterprises and publicise the punishment.

Those who tampered or falsified monitoring data will be taken into custody, according to the ministry.

Inspectors also found that some enterprises failed to take effective measures following alerts, or their measures were impracticable, and that small and scattered factories in Hebei had illegal emission of smoke and dust.

Smog hit parts of northern and central China starting Sunday.

The smog, which engulfed Beijing, Tianjin, Hebei, Shanxi, Henan and Shaanxi, is expected to last until a cold front comes to the rescue around Thursday, according to the National Meteorological Center.

People in affected regions were asked to take precautions and choose public transport services to cut emission and mitigate against pollution.

Severe smog triggered red alerts in more than 20 cities at the beginning of 2017.

When authorities issue red alerts, certain manufacturers are required to cut production and heavily polluting vehicles are banned from roads.

China has a four-tier color-coded warning system for severe weather, with red being the most serious, followed by orange, yellow and blue.