Air pollution in Beijing reached hazardous levels as smog engulfed large parts of the country despite efforts to clean up the foul air.

The US Embassy in Beijing reported the level of the poisonous, tiny articles of PM2.5 at 391 micrograms per cubic metre at noon on Saturday local time.

The World Health Organisation considers the safe level at 25 micrograms per cubic metre of the particulates. Since yesterday, the city had been shrouded in grey smog, reducing visibilities to a few hundred yards.

The Ministry of Environmental Protection has forecast severe pollution for the greater Beijing region, as well as the west part of Shandong province and the northern part of Henan province until Tuesday, when strong winds from the north are expected to blow away air pollutants. The ministry has advised the public to stay indoors.

Authorities blame coal burning for winter heating as a major culprit for the air pollution. The ministry said it had sent teams to check on illegal emissions by factories in several northern Chinese cities.

In the past, authorities have shut down factories and pulled half of the vehicles off the roads to curb pollution.

But such drastic measures are disruptive and only used when Beijing feels it needs to present a better image to the world, such as hosting major global leaders and events.