China's top observatory has raised the severity of its smog alert for northern and central China, with heavy smog expected to continue for another two days.

The National Meteorological Center (NMC) late Monday turned up the smog alert from yellow to orange, the second-highest level in severity, and at 8 a.m. Tuesday continued the alert for another 24 hours, according to a statement.

China's meteorological alerts are labeled blue, yellow, orange and red in order of ascending severity.

From Tuesday to Wednesday, moderate smog will shroud Beijing, Tianjin, Hebei, Shanxi, Shandong, Henan, Shaanxi and Liaoning, where weather conditions will not be favorable for the dispersion of pollutants.

Some of those regions will see heavy smog, the NMC said.

This round of heavy air pollution has persisted since Thursday. Beijing issued an orange alert for the first time on Friday. The alert initiated an emergency response requiring schools and kindergartens to cancel outside sports classes and halting some construction work.

However, the level of PM2.5, particles measuring less than 2.5 micrometers and the most noxious form of air pollution, continued to rise in Beijing, Tianjin and Hebei, the NMC said.

At the air monitoring spot in the National Agricultural Exhibition Center in Beijing, PM2.5 readings topped 400 at 2 a.m. and surged to 444 at 4 a.m. on Tuesday, data showed.

A cold front moving south will help lift the smog starting Wednesday night, the center forecast.

The NMC warned people in affected regions to reduce outdoor activities and take precautions when they have to go outside. Endi