Thick smog enveloped a major Chinese city for a third day Tuesday, with schools and a regional airport shut and poor visibility forcing ground transport to a halt in places.

Images from Harbin, a northeastern city of more than 10 million people and the host of a popular annual ice festival,showed roads shrouded in smog, with visibility in some areas reduced to less than 50 metres.

Flights remained severely delayed, after more than 250 flights were cancelled at the local airport on Monday, according to Chinese media.

Air pollution levels were easing on Tuesday evening but remained as much as 15 times the levels deemed safe by the World Health Organisation.

Figures from monitoring stations showed that concentrations of PM2.5, the tiny airborne particles considered most harmful to health, averaged 247 micrograms per cubic metre in the city, with one station showing 367.

That figure was down from Tuesday morning's level of 822 micrograms per cubic metre and Monday's level of 1,000. The WHO's recommended standard is 25.

The overall air quality index had improved to a measure of 297, or "heavily polluted". Earlier Tuesday, the figure exceeded 500, the highest level on the Chinese scale.

Residents of the far northeastern city described a smog that began choking people as much as a week ago but worsened considerably on Sunday night.

"You could feel the burning smell in the air, and on the second day the thick fog just blocked your way, keeping you from seeing anything," said Song Ting, a 21-year-old student in Harbin. "It's still disgusting."

Zhao Yao, a 25-year-old IT engineer, said: "You feel sick when you breathe. You can't see many people on the street now, and some people wear three masks when going out."

The smog in Harbin came as it activated its public heating system before the icy winter, state media said.

It is China's latest major pollution-related episode.

The issue causes significant public anger and several Chinese newspapers carried images from Harbin on their front pages Tuesday.

In January thick smog blanketed Beijing -- with similar PM2.5 levels to Harbin this week -- garnering headlines, as well as a nickname "airpocalypse", in news reports around the globe.

At the time Harbin escaped the worst of the pollution, but huge areas of northern China have been shrouded by smog on various occasions this year.

Pollution from rapid development and heavy coal use plagues wide swathes of China, prompting public criticism and pledges from the country's leadership to make improvements.

The Harbin pollution came days after details emerged of a plan by Beijing's city government to reinstate an alternate ban on odd- and even-numbered cars when the capital's air pollution reaches red-alert levels for three days or more.

The measure is expected to see an extra two million passenger journeys a day on public transport when it is imposed, city transport official Fang Ping said Tuesday.

Last Friday American jazz singer Patti Austin cancelled a scheduled concert at the Forbidden City Concert Hall in Beijing due to a "severe asthma attack" after arriving in the notoriously smog-ridden capital, according to a statement on her website.