Beijing on Sunday issued its highest-level smog alert this year and urged residents to stay indoors as dangerous air pollution continued to cloak swathes of northern China.

The smog came as global leaders flew to the French capital Paris for a key summit intended to reach agreement on limiting dangerous climate change.

China's capital issued an "orange" alert hours after levels of PM2.5 -- particles which can penetrate deeply into the lungs -- reached more than 15 times recommended levels.

Residents were advised to minimise outdoor activities, while those with respiratory illnesses and the elderly were urged to stay inside their homes.

The US embassy in Beijing on Saturday reported PM2.5 levels at more than 390 micrograms per cubic metre.

Its readings fell below 200 on Sunday, but were still well above the 25 micrograms per cubic metre which the World Health Organization considers safe.

In some parts of Hebei province bordering Beijing, levels reached more than 400 micrograms per cubic metre over the weekend.

Reports linked the spike in pollution to weather conditions and an increase in coal-burning to provide winter heating.

Under the orange alert -- the city's second highest -- industrial plants must reduce production while large trucks are banned from the roads.

China generates most of its electricity and heating by burning coal -- which also contributes to the country's greenhouse gas emissions, the world's largest.

On Sunday hundreds of thousands of people joined worldwide protests aimed at sending a message to the summit.

But Chinese media contained no reports of similar protests. Mass marches are almost unheard of in the country, where the ruling Communist party does not tolerate organised political opposition.