BEIJING, Dec. 19-- China's weather observatory on Saturday issued a yellow alert for smog in the country's north that will linger until Wednesday.

Beijing, Tianjin and neighboring provinces of Hebei, Henan, Shanxi and Shandong, as well as northeast China will face moderate smog, with south Beijing, central Hebei, north Henan and west Shandong to experience heavy smog, the National Meteorological Center (NMC) said.

The smog is forecast to start easing or dispersing from Wednesday in north China, the NMC said.

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

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.