By Jeff Mason and Claudia Parsons

UNITED NATIONS (Reuters) - World leaders tried to inject momentum into climate change talks on Tuesday but new proposals by China and a rallying cry from U.S. President Barack Obama did little to break a United Nations deadlock.

Obama and Chinese President Hu Jintao, leaders of the world's top greenhouse gas polluters, hoped to boost efforts to forge a new global warming treaty two and a half months before a December deadline.

Speaking at a special U.N. climate change summit, Hu laid out a new plan to tackle China's emissions, tying them to economic growth. He did not include specific figures, however. A Chinese official said those would be ready soon.

Obama outlined his administration's work on climate since he took office in January and said the United States was committed to act.

But he offered no new proposals, to the chagrin of environmentalists, and did not urge quick U.S. Senate passage of a climate bill, which many observers see as crucial to reaching an international deal.

The one-day summit drew nearly 100 heads of state and government before official talks among 190 nations in Copenhagen in December to forge a replacement to the Kyoto Protocol, which runs out at the end of 2012.

"While the summit is not the guarantee that we will get the global agreement, we are certainly one step closer to that global goal today," said U.N. Secretary-General Ban Ki-moon at the close of the meeting.

Talks leading to the December 7-18 negotiations in Denmark have put developed and developing countries at odds over how to distribute emissions curbs. Poorer nations are pressing richer ones to contribute hundreds of billions of dollars a year to help them cope with rising temperatures.

Hu said China's new plan included vigorously developing renewable and nuclear energy and promised emissions would grow slower than economic growth in the future.

"We will endeavor to cut carbon dioxide emissions per unit of GDP by a notable margin by 2020 from the 2005 level," Hu said.

The pledge, which marked the first time China has said it will accept measurable curbs on its emissions, was seen as an attempt to counter critics, especially in Washington, who say Beijing is doing too little.

Obama said the United States had done more over the eight months of his presidency to reduce carbon pollution than at any time in history and urged all nations to act together.

"Our generation's response to this challenge will be judged by history, for if we fail to meet it -- boldly, swiftly and together -- we risk consigning future generations to an irreversible catastrophe," Obama said.

"The time we have to reverse this tide is running out."

GROWING IMPATIENCE 

Activists and analysts, while welcoming certain aspects of both men's speeches, expected more.

"It was a bit disappointing that China did not give a number for greenhouse gas intensity," said Knut Alfsen, head of research at the Center for International Climate and Energy Research in Oslo.

"But this is progress. Five years ago, climate was a non-issue for China. Now they ... are saying 'we are going to do something now.' This is a tremendous shift."

Environmentalists criticized Obama for not putting more specifics in his first U.N. address.

"It is really more of a step back than a step forward," said Thomas Henningsen, climate coordinator for Greenpeace International.

Other leaders drew more praise. Japanese Prime Minister Yukio Hatoyama won plaudits for pledging to offer more aid to help developing countries deal with climate change and repeating his goal of reducing Japanese greenhouse gas emissions 25 percent from 1990 levels by 2020.

Europeans, who had welcomed Obama's commitment to fight climate change as a positive development after his predecessor, George W. Bush, have grown impatient.

A climate change bill mandating cuts in U.S. emissions is unlikely to be passed by the U.S. Senate by December while other domestic issues, notably healthcare reform, dominate the agenda.

The U.S. president, who presides over a G20 meeting later this week, noted the climate push came as the world was pulling itself out of recession.

"We seek sweeping but necessary change in the midst of a global recession where every nation's most immediate priority is reviving their economy," he said.

European Commission President Jose Manuel Barroso said countries should press those political constraints, and French President Nicolas Sarkozy proposed that heads of state from big economies meet in November before the U.N. talks. (Additional reporting by Paul Eckert and Alister Doyle; Editing by Doina Chiacu)