"I want to stress that the United States takes climate change very seriously, for we are both a major economy and a major emitter," US Secretary of State Condoleezza Rice said at the start of the two-day conference.

"Climate change is a global problem and we are contributing to it," Rice said. "Therefore, we are prepared to expand our leadership to address the challenge."

Outside the State Department where the sessions were held, dozens of protesters held up anti-Bush placards, "Bush is a criminal" and "Stop Global Warming Now."

"We're here to register our protest at this charade," said Greenpeace USA chief John Passacantando. "President Bush is trying to take the world in the wrong direction on global warming, and this meeting is nothing more than a propaganda effort to deflect international criticism."

Diplomatic security formed a line to stop protesters from entering the building. Nearly 50 demonstrators were arrested.

By most counts, the United States is the world's top emitter of greenhouse gases. But Bush, who rejected the Kyoto Protocol, continues to resist binding targets, calling instead for voluntary approaches and "aspirational" long-term goals.

Rice said individual nations should set their own goals to curb climate-warming emissions, especially carbon dioxide from coal-fired power plants and petroleum-fueled vehicles.

Critics questioned whether such voluntary targets would work.

"We appreciate the sentiments expressed by Secretary Rice, but the devil is always in the detail," South African Environment Minister Marthinus van Schalkwyk told Reuters.

"That is still the crux of the difference between the approach of the US and the approach of the rest of the world," he said, referring to the split over national versus global targets. "For us this meeting is obviously to determine if the US is willing to change (its) approach on that issue."

RICH COUNTRIES URGED TO DO MORE

Chief UN climate change representative Yvo de Boer told the conference he thought the discussions could contribute to the UN process. At a December UN meeting in Bali, Indonesia, representatives will consider a way to cut emissions after the Kyoto pact expires in 2012.

De Boer said it was crucial that industrialized countries commit themselves to an aggressive approach that would involve "going well beyond present efforts, given their historic responsibility and their economic capabilities."

One of the Bush administration's objections to Kyoto was that it exempted fast-growing economies like China and India while penalizing rich countries like the United States.

At least one study this year indicated that China is now the leading emitter of greenhouse gases, ahead of the United States.

A UN meeting on Monday drew more than 80 heads of state and government to focus on the problem of global warming. Bush skipped that meeting although he attended a working dinner hosted by UN Secretary-General Ban Ki-moon.

Other participants at the US climate meeting were the European Union, France, Germany, Italy, the United Kingdom, Japan, Canada, India, Brazil, South Korea, Mexico, Russia, Australia, Indonesia and South Africa.

"I do think this meeting is a deliberate attempt to suggest a very different framework for the new international agreement, one that's based on voluntary measures," said Angela Anderson, vice president for climate programs at the Washington-based National Environmental Trust.

But an Indian official said US official had repeatedly said they were not trying to circumvent the United Nations.

"We are fairly reassured on that point now," he said. "The great thing is the US is engaging on climate change."