Air pollution in Beijing reached "hazardous" levels on Monday, the US embassy said, as thick smog blanketed the city for the third day running, forcing the closure of highways and cancellation of flights.

The Chinese capital is one of the most polluted cities in the world, mainly due to its growing energy consumption -- much of which is still fuelled by coal-fired power stations -- and the high number of cars on the road.

A "hazardous" rating by the US embassy, whose evaluation of the city's air quality often differs markedly from the official Chinese rating, is the worst on a six-point scale and indicates the whole population is likely to be affected.

The embassy has rated Beijing's air quality as hazardous on several occasions this month. On October 9, the reading was listed as "beyond index", meaning it went above measurable levels.

By contrast, China's environment ministry said Beijing's air was just "slightly polluted" on Sunday -- the most recent data available -- sparking a debate in China's state-run media and on the Internet.

Even the usually nationalist Global Times newspaper on Monday demanded an explanation for the disparity, urging the government to "be cooperative in avoiding confusing information" about air pollution.

"Figures by some local governments show the air pollution index is dropping in some cities, such as Beijing... But some Beijing citizens complain the figures do not match their experience," it said in an editorial.

Residents of the capital expressed their fears over the effects on their health on Sina Weibo, a Chinese microblogging site similar to Twitter, with some reporting breathing difficulties and dizziness.

"The pollution reading was again hazardous this morning. We are inhaling poisons," wrote a blogger under the name Xuemanzi.

In April, Beijing launched a five-year action plan to improve the environment by phasing out coal-fired boilers, saying it wanted excellent or good air conditions for 80 percent of the days in the year by 2015.

Authorities said they would refurbish highly polluting coal-fired boilers and stoves in six Beijing districts with equipment that uses clean energy, and also try to phase out 400,000 old, polluting vehicles before the end of 2015.

Beijing's weather bureau issued three heavy fog alerts on Sunday. Six highways closed and over 200 flights were delayed or cancelled as a result, Chinese media reports said.