Beijing's top weather official, Mr Sun Jisong, advised the elderly and young to stay indoors to avoid the risk of contracting respiratory diseases.

'Wear a face mask if you have to go out today,' the official Xinhua news agency quoted him as telling all Beijingers.

Xinhua blamed a heavy fog that had enveloped Beijing for trapping the pollution, and the smog caused havoc across the city yesterday.

Major highways leading into the Chinese capital, one of the world's most polluted cities, were closed, with visibility reduced to 50m in some areas, state press reported.

Thousands of passengers were also stranded at Beijing's Capital International Airport in the north-east as the thick grey haze shrouded the runways and forced flight delays, witnesses said.

The spike in Beijing's environmental problems came a day after International Olympic Committee chief Jacques Rogge said that events at next year's Olympics could be delayed because of pollution.

Mr Rogge, addressing a major environmental forum here, said Beijing had worked hard to improve its pollution woes but could still fall short of ensuring clean air for athletes, especially those competing in endurance events.

'For this reason, we may have to reschedule some events so that the health of athletes is scrupulously protected,' he said.

On Thursday, the United Nations Environment Programme issued a report congratulating Beijing on the US$12 billion (S$17.5 billion) environmental clean-up under way since it won the right to host the Games in 2001.

But it also concluded that the stubborn problem of air quality was unlikely to be resolved in time for the Games.

The UN report said levels of particulate matter - small air-borne particles that can worsen asthma, affect lung function and cause bronchitis - were at times 200 per cent above World Health Organisation standards.

It blamed Beijing's poor air quality in part on the three million cars that clog its streets, a number that is growing by 1,200 daily.

The lead writer of the report, Mr Paolo Revellino, who is visiting Beijing for a few days to take part in the environmental forum, said the pollution yesterday was of concern.

'I looked out the window this morning and I saw all that. I believe it's quite a common sight in Beijing,' he said.

'I'd need my scientific measurements to tell you what it is. But it sure doesn't look too good.'

AGENCE FRANCE-PRESSE