JINAN - Heavy smog has shrouded east China and the public have been warned to take precautions.

At least 13 cities, mainly in Shandong province, will by affected by smog from Friday to Saturday, the provincial meteorological station said.

The station issued a yellow alert through Saturday and warned that the air will not clear until Sunday. Children, the elderly and people with respiratory diseases are advised to stay indoors.

Local farmers' practice of burning corn stalks to dispose of them during harvest season around October has caused lots of smoke, contributing to the smog.

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