The Chinese capital issued its highest red fog alert for a second day on Wednesday, keeping highways closed in and around the city which is already under a smog alert after weeks of choking winter pollution.

A woman wears a face mask on a polluted day in Beijing, China.

China's weather bureau warned of visibility of less than 50 meters in some areas, leading many airports to cancel flights.

More than 2,000 tourists were stranded on a cruise ship for two extra days near the port of Tianjin, as smog prevented the ship from docking until Monday, the Beijing Evening News reported.

Poor visibility prompted three major northern ports to suspend the loading of ships on Tuesday, maritime safety agencies said.

Pollution alerts are common in northern China, especially during bitterly cold winters when energy demand, much of it met by coal, soars. Beijing's smog alert is at the second-highest orange only, raising questions from some living in the city.

The Ministry of Environmental Protection said it hadn't issued a smog red alert because of discrepancies in the forecasts for different parts of Beijing.

China is in the third year of a war on pollution aimed at reversing the damage done to its skies, soil and water after decades of untrammeled economic growth. But measures taken so far have had little or no effect.

Heavily polluted Hebei province, which surrounds most of Beijing, said on Tuesday it had ordered all polluting firms in Tangshan, China's biggest steel-producing city to the east of Beijing, to shut down.

In a statement on its website, the Hebei government named Tangshan Luanxian Xinglong Iron and Steel Co. Ltd as a major culprit for failing to control emissions and even allowing the staff canteen to directly burn coal, contravening regulations.

Hebei, home to seven of China's 10 smoggiest cities in 2015, will also build the world's biggest dust prevention barrier, stretching nearly two miles, at the major coal port of Qinhuangdao in a bid to cut pollution, state media said on Wednesday.

Source: Reuters