BEIJING - A lax enforcement of the law has allowed more construction dust than usual to pollute the city's air, an environmental watchdog said on Monday.

The Beijing Environmental Protection Bureau said that has been in part responsible for the hazy days that have roused the public to anger in the past month.

On Friday, the bureau issued a warning to 17 construction sites that had failed to bring their release of construction dust under control, asking them to eliminate the violations in three days.

The bureau conducted an inspection of several of the sites on Monday and found that none of them had complied with its orders.

Zhao Chengyi, an official with the environmental protection bureau, said the bureau does not have the right to enforce laws, only to supervise what takes place at construction sites.

"Without the right to punish the wrongdoings at these construction sites, we can do nothing but issue warning after warning," Zhao said. "This has become a long-standing problem."

Zhao's remarks followed complaints about the bureau's alleged failure to take steps to improve the city's air quality in response to the heavy fog that shrouded the capital for several days this past month.

Six years ago, a reorganization of government departments gave the Beijing Municipal Bureau of City Administration and Law Enforcement and Beijing Municipal Commission of Housing and Urban-Rural Development the right to enforce environmental protection laws.

The Beijing Municipal Commission of Housing and Urban-Rural Development said on Monday that it is working harder to crack down on releases of construction dust.

The Beijing Municipal Bureau of City Administration and Law Enforcement released no comment on Monday.

In Beijing, more than 5,000 construction sites covering 150 million square meters of space release about 40 percent of the particles in the city's air. They, along with auto exhaust, coal emissions and industrial pollution, are the chief causes of air pollution in Beijing, according to Fang Li, deputy director of the environmental protection bureau.

"If the dust at the construction sites is not well controlled or the dust on the ground is not hardened, it can be easily blown into the air and increase the particle concentration," Fang said. "It's especially hazardous to people's health on dry winter days."

According to Wang Xiaoming, another official with the environmental protection bureau, fighting construction dust is one of the bureau's priorities.

"We're paying attention to the dust pollution created by construction in residential areas, downtown and at some large construction sites in the capital," Wang said. "Those companies all know the regulations, but they simply don't follow them."

Some think the pursuit of profits is driving their actions.

"To have the roads hardened at this construction site, which covers about 1,000 square meters, we need to invest at least 300,000 yuan ($47,000), which is a lot," said a staff worker at Haohai Construction Group, which is building the second phase of the Zhongguancun Science and Technology Park. The staff worker declined to give his full name.

The project, designed as an incubator for high-tech businesses, has released large amounts of dust and the company has been the subject of many warnings from the environmental protection bureau. The bureau's inspection on Monday found that no improvements had been made to the site.

A company that does not clean up construction dust will be fined 20,000 yuan at the most, according to the Beijing Municipal Bureau of City Administration and Law Enforcement.

The environmental protection bureau is now asking the public to help it supervise construction sites. A reward of up to 300 yuan will go to those who report large cases of dust pollution.

"We want to fully mobilize the public," said Li Hua, head of the bureau's environmental protection monitoring team.