"The conflict between environment and development is becoming ever more prominent," the paper said.

"Relative shortage of resources, fragile ecology and insufficient environmental capacity are becoming critical problems hindering China's development."

China's Communist Party has promised to balance economic development with environmental safeguards, after a year in which sandstorms that coated the capital in dust and a chemical spill that poisoned a major river thrust its record on pollution into the global spotlight.

But economic growth has been averaging about 10 percent annually, far greater than the 7.5 percent growth rates on which the government's targets for pollution reduction are based.

"If the economy is growing too rapidly, environmental resources will be faced with tremendous pressures and therefore such development is not sustainable," Zhu Guangyao, a vice-minister at the State Environmental Protection Administration, told a news conference.

Considerations of land and other resources would play a more prominent role in the approval process for large-scale projects, Zhu said. Such projects have sometimes led to riots over environmental pollution in recent years.

Zhu said implementing the central government's guidelines would also be a challenge for local officials who are accustomed to being judged on growth above all else and are fearful of the economic impact of tighter environmental controls.

"Local environmental NGOs do not dare criticise local governments for their unscientific decisions," Zhu said. "Some local governments are reluctant to implement or are even working against environmental laws."

To mark World Environment Day, in Beijing, where 1,000 new vehicles take to the road every day, a quarter of a million people pledged to leave their cars at home, state media reported.

But in the city of more than 16 million, there was little discernable impact on the capital's smoggy skies.

And in Shanghai, China's richest city, the local environment bureau was quoted as saying that companies that pollute would find it harder to get loans as their green credentials would be linked to their creditworthiness.

But China's enforcement is still dogged by under-funding and a lack of resources, a senior SEPA official was quoted as saying on the ministry's Web site (www.zhb.gov.cn).

"Investment in ecological protection is inadequate," the official said. "We need to adopt more vigorous measures."