BEIJING, Jan. 15 (Xinhua) -- Smog will hit some parts in north and central China starting from Sunday, China's national observatory warned, suggesting people in affected areas to make precautions.

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

People in the affected regions were asked to make precautions and choose public transport services to cut emissions and mitigate pollution.

Severe smog triggered red alerts in more than 20 cities at the beginning of 2017. When authorities issue red alerts, some manufacturers are required to cut production, and heavily polluting vehicles are banned from the 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.

In environment inspections last year, the Ministry of Environmental Protection said a total of 720 people were detained and 6,454 held accountable for environment-related wrongdoing.

In 2016, the government refused 11 high-pollution and high-energy consuming projects, involving total investment of 97 billion yuan (about 14 billion U.S. dollars).

Partly due to those efforts, Chinese cities reported less pollution in 2016, with the average density of PM 2.5 -- fine particles that pose health risks -- in 338 cities falling by 6 percent.

China is aiming for a 10-percent reduction in air emissions from 2012 levels by 2017 in cities at the prefecture level and above. Meanwhile, the PM 2.5 density in Beijing, Tianjin and Hebei Province should drop 25 percent.