A heavy smog envelops Beijing. (Xinhua Photo)

A heavy smog that descended on Beijing Monday continued to envelop the capital city early Tuesday, causing highways to close and affecting dozens of flights.

Visibility in Beijing was less than 1,000 meters and in part only 500 meters on Monday, according to the Beijing Meteorological Observatory, which issued smog warnings to motorists and the public.

Sixteen flights were delayed and nine cancelled by Monday evening at Beijing Capital International Airport, sources with the airport said.

Highways from Beijing to Tianjin, Shenyang, capital of northeast China's Liaoning Province, Shijiazhuang, capital of neighboring Hebei Province, and Kaifeng City, central China's Henan Province, have been closed due to the smog.

The smog also caused serious air pollution, making Monday the most polluted day so far this month, according to the Beijing Environmental Protection Monitoring Center.

People were advised to reduce outdoor activities as much as possible and polluters asked to take measures to reduce emissions, according to the bureau.

Fog and pollutants that could not be diffused due to low air fluidity in the past few days resulted in the smog, said meteorologists, who forecast the smog may disappear late Tuesday.

Editor: Donald