UNITED NATIONS - President Barack Obama challenged the world on Tuesday to act swiftly to fight global warming but offered no new proposals that could jumpstart stalled talks on a U.N. climate pact.

Speaking shortly after Obama at a special U.N. summit on global warming, Chinese President Hu Jintau pledged to reduce the carbon intensity of his country's economic growth.

In his speech, Obama said time was running out to address the problem.

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, he said.

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

Activists hoped the United States and China would inject momentum, 2-1/2 months before 190 nations gather in Copenhagen aiming to complete a deal to slow climate change.

U.N. Secretary-General Ban Ki-moon, who called the meeting, said talks were moving too slowly.

Failure to reach broad agreement in Copenhagen would be morally inexcusable, economically short-sighted and politically unwise, Ban said.

We cannot go down this road. If we have learned anything from the crises of the past year, it is that our fates are intertwined, he said.

Talks leading to the December 7-18 meeting 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.

European Commission President Jose Manuel Barroso said talks are dangerously close to deadlock and were in danger of an acrimonious collapse.

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

An aggressive move by China to curb its emissions, even if short of an absolute cap, could blunt criticism in Washington, where many lawmakers are reluctant to commit to U.S. emission cuts without evidence that Beijing is acting.

Obama's legislative initiatives to reduce U.S. emissions have been overshadowed by his push for healthcare reform. But he said in his speech that the United States had done more over the last eight months to reduce carbon pollution than at any time in history.

(Editing by Alan Elsner)

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