JINAN - More than 200 expressway toll gates in east China's Shandong Province were closed on Monday due to a persistent smog since Saturday.

The province issued a yellow alert on Monday morning as visibility in parts of the region dropped below 200 meters. The heavy smog is likely to continue in the morning, according to the provincial meteorological center.

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

Smog has shrouded much of northern China including Beijing, Tianjin, Hebei, Henan, Shandong, Shanxi and Shaanxi.

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