A photo shows heavy smog engulfs Beijing on November 30, 2015. [Photo: Xinhua]

Smog worsened in north China on Monday as the meteorological center upgraded its alert from yellow to orange, the second most serious alarming.

Smog will continue in most parts of north China on Tuesday and Wednesday, the National Meteorological Center (NMC) forecast.

Serious pollution, along with smog, hit Beijing, Tianjin, Hebei, Shandong, Shanxi and Henan on Sunday and Monday.

The public has been warned to take protective measures when engaging in outdoor activities and those that suffer from respiratory diseases are advised to stay indoors.

The NMC issued a yellow alert on smog on Saturday and Sunday.

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