Tens of millions of people across northern China were under severe pollution warnings Tuesday, with authorities urging them to reduce outdoor activities as a greyish smog enveloped the region.

Several areas of north China's Beijing-Tianjin-Hebei region, home to over 100 million people, are suffering from some of the worst pollution they have seen in months.

In the capital Beijing, authorities Monday issued an orange alert — the country's second-highest pollution warning.

"There has been more smog in the past two days… it really has an effect," said office worker Fu Dalin, 27.

"Because the smog is so serious, I won't go out for a run like usual, and will just have to stay at home," he told AFP in a business district in downtown Beijing.

Another resident of the capital, Xu Gengying, said the pollution wouldn't stop her from going about her everyday life.

"We still go out when we need to, if there's something to do. Just avoid doing some exercise outdoors — no jogging," she said.

In several areas in densely populated Hebei province, which surrounds the capital, authorities issued their highest red alert.

Air quality monitoring firm IQAir on Tuesday listed Beijing as the fifth most-polluted major city on Earth, just ahead of Indian megalopolis Mumbai.

Authorities urged residents to "to reduce going out and strenuous outdoor exercise in the near future," according to state news agency Xinhua.

They were also limiting the number of construction and transport vehicles allowed on the roads.

In parts of Hebei, one official forecast showed that hazy conditions brought visibility down to lower than 50 metres.

In the nearby city of Tianjin, south of Beijing and home to 15 million people, the meteorological bureau advised anyone suffering from respiratory issues to refrain from going outside and to wear a mask if they do.

'Unfavorable conditions'

Official forecasters say the smog, which they blamed on "unfavorable weather conditions", will linger in the region until temperatures drop from unseasonably mild highs later this week.

Should a red alert be issued in Beijing, that would trigger measures such as reducing traffic by prohibiting emitting vehicles with license plates ending in even and odd numbers from driving on alternate days.

The Chinese capital declared "war on pollution" after winning the Winter Olympics bid in 2015, shutting down dozens of coal plants and relocating heavy industries to shed its status as one of the world's most polluted cities.

But while there has been improvement in recent years, air quality remains far below World Health Organization standards.

China is the world's biggest emitter of the greenhouse gases driving climate change, such as carbon dioxide.

Greenpeace said in April that China has approved a major surge in coal power this year, accusing it of prioritizing energy supply over its pledge to reduce emissions from fossil fuels.

The jump in approvals for coal-fired power plants has added to concerns that China will backtrack on its goals to peak emissions between 2026 and 2030 and become carbon-neutral by 2060.