Media playback is unsupported on your device Media caption China smog shuts down city

Smog has choked China's northern city of Harbin, Heilongjiang province, forcing schools and highways to shut and disrupting flights on Monday.

The density of airborne particles was several times above World Health Organisation recommended limits.

Visibility was reduced to below 50m (160ft) in parts of the city, reports said.

Local media have linked the pollution to the city switching on its public heating system for winter.

PM2.5 levels, used to measure the amount of particulate matter smaller than 2.5 micrometres in the air, were above 500 micrograms per cubic metre on Monday morning, state-run news agency Xinhua reported.

World Health Organization guidelines say average daily concentrations of PM2.5 should be no more than 25 microgrammes per cubic metre.

All of Heilongjiang province's highways, and the Taiping International Airport in Harbin were forced to close, Xinhua reported.

A red alert for thick smog had been issued in the provinces of Heilongjiang, Jilin and Liaoning, the news agency added.

Earlier this year, air pollution in China's capital, Beijing, also soared past hazardous levels.

Image caption Cars and buildings are barely visible in the smog that has shrouded Harbin

Image caption People donned masks before venturing outside

Image caption Some highways and bus services were forced to close as a result of the smog

The smog generated extensive discussion on Sina Weibo, one of China's Twitter-like microblogs.

"We were all late for class today because we couldn't find the academic building," microblog user MaltzZz from Jilin province wrote.

Harbin-based Sina Weibo user Backpacker Xiao described Harbin as "today's dead city".

"Beijing, you're no longer alone. You have us too now," he wrote, in reference to the frequent air pollution suffered by Beijing.

Meanwhile, netizen Pen and Ink Silent, a student in Changchun, Jilin, posted the following tongue-in-cheek message: "My mother called... and asked how the air in Changchun was, and if I was coughing. I feel ok, I should be able to live to see my graduation."