Beijing (dpa) - City schools closed Tuesday as Beijing issued its first-ever smog red alert, advising residents to stay indoors and children not to attend classes until Thursday afternoon.

The new alert issued by the city's emergency management headquarters represented the highest-level warning issued for the first time since a four-colour scale - red, orange, yellow, blue - was introduced in 2013.

Education authorities ordered kindergartens, primary and secondary schools to suspend classes as thick haze engulfed the streets and rendered some skyscrapers invisible.

Residents were advised to stay indoors as much as possible from 7 am Tuesday to 12 noon Thursday by the Beijing environmental protection bureau.

The alert was upgraded to red on Monday evening. It reflects a forecast of three days of PM2.5 - airborne particulate matter 2.5 microns and smaller - levels above 200 micrograms per cubic metre.

The US embassy's air quality index showed PM2.5 levels as high as 365 micrograms per cubic metre.

The World Health Organization's recommended safe maximum is 25 micrograms.

Beijing has suffered more serious smog lasting more than three days in the past, but it was not forecast and so did not trigger the red alert.

Emergency measures include restricting road traffic, except electric vehicles, using an odd and even licence plate number system pioneered before the Beijing 2008 Olympics.

With motor vehicle use limited, the capital city must transport an extra 2 million passengers riding by bus, rail, subway and train, the city's taffic commission estimated.

More than 150 buses have been added to address the demand, the State Council Information Office said.

Some industrial plants and construction sites were told to pause operations. Fireworks and open-air barbecues are also banned by the alert.

Beijing made global headlines last week during the Paris climate change talks when its air quality went off most scales with a PM2.5 reading of 678 near Tiananmen Square.

PM2.5 pollution is fine enough to penetrate deep into the lungs, and is associated with increased risk of heart attack, stroke, lung cancer and asthma.

Beijing may struggle to clean up its air in time for the 2022 Winter Olympics, as the sources of its air pollution are not local, according to US researchers.

The capital city "receives much of its pollution from distant industrial areas, particularly Shijiazhuang," 322 kilometres to the south-west, said Robert Rohde, co-author of a report on China's pollution.

Bad air contributed to 1.6 million deaths a year or roughly 17 per cent of all deaths in China, according to a report by Rohde and co-author Richard Muller, in the science journal PLOS One.

Air pollution kills about 4,000 Chinese people a day, with coal burning a key contributor, it said.

The smog is expected to dissipate when a cold front arrives on Thursday afternoon, according to the city's Environmental Monitoring Centre.