A visitor takes a snapshot of the smog-filled sky in Beijing, Dec 8, 2015. [Photo/IC]

Beijing's environmental watchdog issued on Monday for the first time its highest-level air pollution alert, with the capital facing a second wave of smog this winter.

The smog also threatens a large area of northern China with at least nine other cities, including Tianjin and eight cities in Hebei province.

The unprecedented red alert will be effective from 7 am on Tuesday until 12 am on Thursday.

It imposes the toughest restrictions on activities that may contribute to the pollution worsening, including a ban on the use of half of the city's cars-nearly 2.5 million vehicles a day.

The heavily polluted cluster of Beijing, Tianjin and Hebei will experience windless weather that could keep pollutants accumulated near ground level. Air pollution levels may remain hazardous until Wednesday, the municipal air quality monitoring station said.

A researcher at the China Research Academy of Environmental Science, an environmental think tank, said Beijing's red alert may be followed by governments in neighboring cities when necessary and a joint effort by all environmental authorities across the Beijing-Tianjin-Hebei area to control pollution.

Beijing was already under an orange alert－second to red－on Monday. Forecasters say fresh winds will not arrive until Wednesday to change the weather conditions and disperse the pollutants.

But there is little room for optimism, as more smog may be on the way, which could affect Beijing from Dec 14 to 16, according to the National Meteorological Station.