(Updates with de Boer, South Africa, US Congress, edits)

By Jeff Mason and Deborah Zabarenko

WASHINGTON, Sept 28 (Reuters) - Some of the world's biggest greenhouse polluters took aim at President George W. Bush on Friday, calling him "isolated" and questioning his leadership on the problem of global warming.

Bush, who convened the two-day meeting of the 17 biggest emitters of climate-warming gases, stressed new environmental technology and voluntary measures to tackle the issue.

"Our nations have an opportunity to leave the debates of the past behind and reach a consensus on the way forward and that's our purpose today," Bush told an audience that included delegates from Europe, Japan and Australia as well as fast-growing developing countries such as China and India.

But his speech did little to dampen doubts from participants and environmentalists that the climate session at the State Department would help advance crucial U.N. talks in Bali, Indonesia in December.

"It is striking that the (Bush) administration at the moment in the international conversation seems to be pretty isolated," said John Ashton, Britain's climate envoy. "I think that the argument that we can do this through voluntary approaches is now pretty much discredited internationally."

Bush's rejection of mandatory limits on carbon dioxide and other greenhouse gases that warm the planet is at odds with the 1997 Kyoto Protocol and with many who attended on Friday.

"Our message to the U.S. is this: what they placed on the table at this meeting is a first step, but is simply not enough," South African Environment Minister Marthinus van Schalkwyk said in a statement. "We think that the U.S. needs to go back to the drawing board."

The United States has long been the world's biggest greenhouse emitter but at least one study this year put China in the lead. Given this U.S. role in contributing to the problem, van Schalkwyk said the United States should contribute its "fair share" to a solution.

LOOKING TOWARD BALI

By mid-2008, Bush said heads of state of the biggest emitting countries should set a long-term target to fight climate change, and said there should be "a strong and transparent system for measuring our progress toward meeting the goal we set."

This drew a muted response from delegates, according to Yvo de Boer, the special U.N. envoy on climate change.

De Boer said he found Bush's speech "encouraging" because it acknowledged the urgency of the issue.

But asked to predict the outcome of the Washington meeting, de Boer replied, "The very strong indication I got is that people said, 'This is a very interesting discussion but we need to continue it after Bali.'"

In fact, delegates applauded when Bush stressed that this meeting was meant to lay the groundwork for the Bali conference. Some critics have questioned whether the Bush administration was attempting to get around the U.N. climate process with its own set of meetings.

Many of Bush's allies in Europe and Asia have been pushing the United States to embrace an approach of mandatory targets such as those embodied in the 1997 Kyoto Protocol, which Bush rejected.

U.S. Democrats criticized Bush's stance, with presidential candidate Barack Obama, an Illinois Democrat, calling the Washington meeting a "missed opportunity." He said Bush's "voluntary measures reflect neither innovation nor leadership."

Sen. Jeff Bingaman of New Mexico, who co-authored a bill to curb emissions, said the United States "will lack the credibility to negotiate any serious international climate agreement unless it can come up with a credible plan to limit greenhouse gas emissions domestically."

Bush said a long-term goal for reducing global warming is needed, but said each nation should design its own strategy. He suggested that a global clean-technology fund could be led by U.S. Treasury Secretary Henry Paulson, to be financed by global contributions.

The Bali talks will aim to launch a successor to the Kyoto Protocol, a treaty that set limits on industrial nations' emissions; its first phase ends in 2012.

(Additional reporting by Caren Bohan)

(Editing by Vicki Allen; jeff.mason@reuters.com, Reuters messaging: jeff.mason.reuters.com@reuters.net))

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