Poor visibility delayed more than 180 domestic and international flights serving Beijing on Wednesday, the capital's airport said, following several days of heavy air pollution.

"As of 10 am today, 180 flights in and out of the airport experienced delays," an official in the information centre of Beijing Capital Airport, who declined to give his name, told AFP.

The flight woes followed several days of poor air quality in the city, underlining China's stubborn environmental problems even as the government plays up its eco-awareness ahead of a global summit on climate change.

A number of flight delays and cancellations also were reported by various state media in other parts of the country, some of which were blamed on locally poor air quality.

Official Beijing government data widely viewed as downplaying the city's pollution woes have shown air quality readings spiked to "slightly polluted" in recent days.

An airport statement attributed the poor visibility to "a heavy attack of fog" that reduced visibility to as little as 50 metres (yards). "Fog" is typically used by the government to describe haze from air pollution.

However, an air quality index published on social networking site Twitter by the US embassy as an alternative to official data has catalogued long stretches of "hazardous" pollution levels in recent days.

One of the world's most polluted countries, China has increasingly stressed its commitment to changing its dirty ways ahead of a December 7-18 climate change summit in Copenhagen.

President Hu Jintao told the United Nations in September that China would reduce the intensity of its carbon emissions as a percentage of economic growth by a "notable margin" by 2020 from their 2005 levels, but gave no figure.

But air pollution remains stubborn, with smog once again a familiar sight in Beijing, dashing hopes that aggressive clean-up steps taken for the Olympic Games last August would have lasting benefit.