An environment official has pledged to maintain Beijing's air quality "on good or moderate levels" during the Olympic Games, denying the recent haze over the city was caused by pollution.

"Keeping an eligible air condition for the Olympic Games is a promise made by the Chinese government, and we will honor our commitment," Du Shaozhong, deputy director of Beijing Municipal Environmental Protection Bureau, told a press conference in Beijing yesterday.

The Chinese capital has been fighting air pollution since 1998, trying to reduce smog mainly created by emissions and construction sites.

"Beijing has made three major leaps in improving the air quality," said Du. He said that, from 1998 to 2007, the number of days on which the average air pollution index (API) was under 100 (the Chinese standard) had increased from 100 to 246.

Major pollutants such as sulfur dioxide, carbon monoxide and carbon dioxide had been controlled within national standards in that time, he added.

The city has taken a series of measures since last year to clean the air. Hundreds of factories on the outskirts of Beijing which produce heavy emissions have been temporarily or permanently shut.

Shougang Group, one of the country's biggest steel makers, and several coking plants have been moved from the city. Construction sites have been ordered to suspend work before the Games start on August 8.

Since July 20, the capital's three million cars have been forced to operate only on alternate days based on even or odd licence-plate numbers.

"Monitoring analysis shows the emergency measures adopted since July have helped improve the air quality," the deputy director said, adding that there had been 22 days with an API under 100 between July 1 and 25.

However, Beijing has been covered by haze since Friday.

"Good air quality does not necessarily mean blue sky," Du said. "We should judge whether there is pollution by scientific statistics, not by what the eyes can see."

"Technically, the haze was partly caused by rainy weather," Guo Hu, head of the Beijing Municipal Observatory, told the conference.