"This phenomenon does not exist," Du Shaozhong, Beijing Environmental Protection Bureau spokesman, told a news conference.

"This is a misunderstanding. We have noticed these reports and we think they are unfair."

Du said Beijing was constantly improving its air quality data gathering techniques and a breakdown of the data from each individual station was publicly available on the bureau's Web site (www.bjepb.gov.cn/air2008).

Pollution remains one of the biggest problems facing Beijing in the run-up to the Aug. 8-24 Games and China has invested some 120 billion yuan ($16.80 billion) over the last decade in environmental improvements.

Many athletes have expressed concerns about air quality in the Chinese capital and International Olympic Committee (IOC) president Jacques Rogge said last year that some endurance events could be rescheduled if the air was too polluted.

Du, who was introducing the 14th phase of Beijing's air pollution control measures, said great progress was being made in clearing the skies of one of the world's most polluted cities.

"I must tell you a very important fact that the air quality this January and February has shown further obvious improvements on previous years," he said.

Du said the implementation of the 14th phase would reduce the emissions of 50,000 tonnes of inhalable particles, sulphur dioxide, nitrogen oxide, volatile organic compounds and carbon monoxide.

Specific contingency measures to ensure clean skies during the Olympics and September's Paralympics, including traffic control measures, would be announced later in the year, he added.

Wang Jian, director of the division of Air and Noise Pollution Control at the State Environmental Protection Agency (SEPA), told Reuters last week that six provinces had been ordered to cut emissions for two months from late July.

Beijing, Tianjin, Hebei, Inner Mongolia, Shanxi and Shandong had all been given detailed targets to reduce pollution using air-cleaning technology or partial closures rather than the wholesale shutdown of industry, Wang said.

The plan will come into operation at the opening of the Olympic Village on July 27 and end when it closes after the Paralympics on Sept. 20, he added.

Du conceded that weather patterns were a big factor in pollution control and officials, even with all their measures in place, would be keeping their fingers crossed that there would be favourable conditions in August.

"To improve the air quality in Beijing needs both people's effort and a little luck" he said.

Take a look at the Countdown to Beijing blog at http://blogs.reuters.com/china