The decision follows last year's ruling by the US Supreme Court, which ordered the agency to devise ways to regulate greenhouse-gas emissions from motor vehicles under the Clean Air Act.

But the EPA said in a 588-page report released Friday that given "the complexity and magnitude of the question" there were doubts whether "greenhouse gases could be effectively controlled under the Clean Air Act."

EPA Administrator chief Stephen Johnson said that rather than attempt to forge a consensus "on matters of great complexity, controversy, and active legislative debate," he had decided to publish the views of other agencies and to seek comment on them during a 120-day review period.

The delay, observers indicate, means that any substantive regulatory action will be almost certainly left to the next administration.

"One point is clear: the potential regulation of greenhouse gases under any portion of the Clean Air Act could result in an unprecedented expansion of EPA authority that would have a profound effect on virtually every sector of the economy and touch every household in the land," Johnson wrote.

In a political blow to US President George W. Bush, the Supreme Court ruled in April 2007 that the EPA must consider greenhouse gases as pollutants and deal with them.

The ruling came in response to legal action undertaken by Massachusetts and a dozen other states and environmental groups that went to court to determine whether the agency had the authority to regulate greenhouse gases such as carbon dioxide emissions.

The Bush administration has fiercely opposed any imposition of binding emissions limits on the nation's industry and has refused to ratify the Kyoto Protocol on reducing greenhouse gas blamed for global warming.

Environmentalists have alleged that since Bush came to office in 2001 his administration has ignored and tried to hide looming evidence of global warming and the key role of human activity in climate change.

At a hearing in November 2006, Massachusetts argued that it risked losing more than 4.5 meters (15 feet) of land all along its coastline if the sea level should rise by 30 centimeters (one foot).

But the Bush administration, backed by nine states and several auto manufacturers, urged the court not to intervene, arguing that if the situation was so dire it could not be solved by a simple legal decision.

It further argued that reducing emissions from new US motor vehicles would have only a minor effect on global climate change.

While the court's decision is unlikely to change US policy, it has ramifications on several other ongoing issues, such as the agency's refusal to regulate emissions from electricity plants which produce some 40 percent of US carbon dioxide emissions. Motor vehicles are responsible for just 20 percent.

The EPA's decision to again delay action has sparked sharp criticism from congressional Democrats.

"The Bush administration decision today to effectively reject regulation of global warming pollution under the Clean Air Act creates a clear and present danger to the American people," said Democratic Senator Barbara Boxer of California, who chairs the Senate Environment and Public Works Committee.

"Despite the Supreme Court's finding that EPA was ducking its responsibility under the law to control global warming emissions, the Bush administration continues to block all action," she added.

David Baron, a managing attorney at the environmental group Earthjustice, said the world community needed action, not just more talk against global warming.

"The administration is fiddling while the planet melts," he said. "If these delays drag on, we will go back to court to force real action."