Beijing residents were urged to stay indoors Thursday as pollution levels soared before a sandstorm brought further misery to China's capital.

A thick blanket of smog covered large swathes of the country in the morning, causing residents to once again dig out face-masks as China's gruelling winter of pollution continues.

The noxious haze saw the US embassy's air quality index reading for Beijing hit 516 at 6am, signalling air quality worse than the highest classification of "hazardous".

Those who ventured out in mid-morning were confronted with swirling clouds of dust, which the Beijing Municipal Environmental Monitoring Center said had been blown in from Inner Mongolia.

"We would hope that everyone stays indoors as much as possible and that people carry out appropriate measures for protection," the agency said in a posting on its verified account on Sina Weibo, China's version of Twitter.

State broadcaster China Central Television showed images of tree branches being blown onto Beijing's streets, and the newsreader urged residents to keep windows closed because of the risk of windows being blown out and showering pedestrians with glass.

The weather was also the subject of resigned discussion on China's Internet message boards, which attracted widespread anger in previous bouts of heavy smog.

"We have gone from toxic pollution to dust pollution," said one poster on Sina Weibo. "We lead a really varied life in Beijing."

Many parts of China have endured repeated episodes of toxic air in recent weeks, sparking demands for government action from both netizens and state media.

Air quality improved during the day, with the US Embassy index reaching 168 at 1pm.