Haze-shrouded Beijing could restrict more cars and shut more factories if air pollution persists during the Olympic Games, a report said on Monday, as an environmental group said endurance athletes could face problems.

Beijing has ordered many cars off roads and halted much construction and factory production in an effort to cut smog before the Games open on August 8.

But the city has still endured hazy skies over the past week, and again on Monday, raising fears that the sultry heat Beijing often experiences in August could make for a cocktail of haze, fumes and dust for tens of thousands of athletes and visitors.

An official newspaper indicated authorities may take more drastic steps to choke off pollution.

"More vehicles could go off the roads and all construction sites and some more factories in Beijing and its neighboring areas could be closed temporarily if the capital's air quality deteriorates during the Olympic Games," the China Daily said.

An official with the Beijing Environmental Protection Bureau confirmed that plans for more cuts were being studied, but he declined to give details.

"The measures will be put into place if air quality turns out to be short of the standards we have promised," the official, who would only give his surname Zhang, told Reuters by telephone.

Pollution has been one of the biggest worries for Games organizers who have said they may reschedule endurance events to prevent health risks to athletes.

The environmental group Greenpeace mixed praise for Beijing Olympic organizers' energy- and water-saving efforts with warnings that air pollution, especially particulate matter, remains a long-term problem.

Beijing's measures of PM10 - particulate matter 10 microns in diameter, about a seventh the thickness of a human hair - have consistly stayed above Chinese national standards and stricter World Health Organisation (WHO) standards, Greenpeace said in an assessment of the Beijing Games.

Lo Sze Ping, campaign director of Greenpeace China, told a news conference that Games organizers and "sports teams from the various countries have reason to be concerned."

"In the event PM10 conditions cannot meet with national standards and WHO standards, then there are reasons for different parties to suggest that certain events be delayed," said Lo.

For four days now, Beijing had not experienced a "blue sky day," when the air pollution index stays below the national standard for "good air quality."

Cars are now banned on alternate days depending on their license plate number - odd or even - and many government cars have been ordered off the roads. Taxis, buses and Olympic vehicles are exempt. Around Beijing, heavily polluting factories, such as steel plants, have also been closed.

A city environment official said last week that air was improving, with a 20 percent cut in carbon monoxide, nitrogen dioxide and particulate matter since the same time last year.

But Greenpeace gave Beijing a mixed assessment. It noted energy-saving technology in Olympic venues, stricter vehicle emissions standards and expanded public transport.

But it said clean-up efforts were hampered by lack of policy transparency and independently verified data. Beijing did not even officially collect statistics on smaller particulate matter and ozone, pollutants that worry health experts, Greenpeace said.

"Beijing has also missed a golden opportunity in using the Games as a platform to implement more ambitious initiatives," said Lo.