UNITED NATIONS - China and the United States, the world's top greenhouse gas emitters, will try to ignite efforts on Tuesday to secure a U.N. global warming pact as worries grow of a dangerously close deadlock in talks.

U.S. President Barack Obama and Chinese President Hu Jintao address a special U.N. summit just 2-1/2 months before representatives from 190 nations gather in Copenhagen to negotiate a deal to combat climate change.

Talks leading to the December meeting have not gone well.

Developed and developing countries are at odds over how to distribute emissions curbs while poorer nations press richer ones to contribute huge sums of money to help them cope with rising temperatures.

Obama and Hu, who are scheduled to meet one-on-one after the summit, could help break the climate impasse.

The Chinese leader, whose country is the world's biggest emitter of carbon dioxide and other climate-warming gases, is expected to lay out new proposals that may include a carbon intensity target -- a pledge to cut the amount of greenhouse gasses produced for each dollar of national income.

This suite of policies will take China to be the world leader on addressing climate change, said Yvo de Boer, the U.N. climate chief, on Monday, anticipating the announcement.

An aggressive move by China to tackle its emissions -- even if short of an absolute cap -- could blunt criticism by leaders in Washington, many of whom are reluctant to commit to U.S. emission cuts without evidence that Beijing is doing the same.

Obama, whose legislative initiatives to reduce U.S. emissions have been overshadowed by his push for healthcare reform, will try to fulfill his promise of showing leadership toward getting a global deal, even as chances that the U.S. Senate will pass a climate bill by December dim.

Martin Kaiser, climate policy director for environmental group Greenpeace International, said the president had allowed vested interests to undermine his promises so far.

This is Obama's opportunity to be a global leader and signal to the rest of the world that the US will take on its fair share of the effort to cut greenhouse gas emissions over the next 10 years, Kaiser said in a statement.

IN NEED OF MOMENTUM

Tuesday's meeting, called by U.N. Secretary-General Ban Ki-moon, will gather nearly 100 heads of state and government to focus on climate. Though it is not a negotiating session, activists hope momentum from the meeting will trickle down to the actual talks.

I hope world leaders will leave the Summit ready to give their negotiating teams the green light and specific guidance needed to accelerate progress on the road to Copenhagen, Ban said in a statement.

The clock is ticking. I hope they will publicly commit to sealing a deal in Copenhagen, he said.

The European Union, which welcomed Obama's more aggressive stance on climate policy compared to his predecessor George W. Bush, has become increasingly frustrated with the U.S. administration's lack of progress.

If we don't move this week, there is a real risk that we will miss the opportunity in Copenhagen, European Commission President Jose Manuel Barroso told reporters after a speech in which he described U.N. talks as being dangerously close to deadlock.

Europe wants rich countries among the Group of 20 to find some $10 billion annually for the developing world as an advance payment toward reaching a climate deal this year.

G20 leaders are expected to discuss the issue in Pittsburgh later this week, but, barring a breakthrough in the U.N. summit, little progress is expected.

(Additional reporting by Timothy Gardner and Walter Brandimarte; Editing by Eric Beech)

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