Poor visibility delayed about 280 domestic and international flights serving Beijing on Wednesday, the capital's airport said, following several days of heavy air pollution.

"As of 3 pm today, about 280 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 nation's air transport hub, underlining China's stubborn environmental problems even as it plays up its eco-awareness ahead of a global summit on climate change.

An airport statement attributed the poor visibility to "a heavy attack of fog" that reduced visibility to as little as 50 metres (55 yards). "Fog" is typically used by the government to describe haze from air pollution.

Flight delays, cancellations and even road closures also were reported by state media in other parts of the country, some of which were blamed on locally poor visibility.

Fog caused a four-hour closure of the main airport in Chengdu, capital of southwestern Sichuan province in the morning, delaying 105 flights and stranding more than 10,000 passengers, Xinhua news agency said.

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

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 problems such as air pollution remain stubborn due to China's rapid industrialisation in recent decades, prioritisation of economic growth over environmental protection and soaring sales of cars and other pollution sources.

Amid fears that chronic smog could damage athlete's health, Beijing moved or cleaned up polluting factories, introduced driving restrictions, and ordered a halt to all construction work during the Games.

The city has retained less-stringent driving curbs and also introduced new subway lines touted partly for their long-term environmental gains, but smog has once again become a familiar grey blot on its skyline.

State media reports said in September that sales of new cars in Beijing had spiked to about 2,000 a day amid rising incomes, a trend expected to put up to four million vehicles on its streets by year's end.

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