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.

Flights and some bus services have been postponed

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.

(Source: BBC)