Barack Obama, the US president, has said the world must address climate change now or suffer an "irreversible catastrophe", at a high-level conference on climate change at the United Nations headquarters in New York.
Ban Ki-moon, the UN secretary-general, on Tuesday opened a climate summit of about 100 leaders saying that failure to reach a new treaty this year on fighting global warming would be "morally inexcusable".
Obama, in what was his first speech at the UN, said that the United States was "determined" to act on global warming, but offered no new proposals that could
jumpstart stalled talks on a UN climate pact.
"Though many of our nations have taken bold actions and share in this determination, we did not come here today to celebrate progress. We came because there is so much more progress to be made.
"We came because there is so much more work to be done.
"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."
Ban called on presidents, prime ministers and other leaders "to accelerate the pace of negotations and to strengthen the ambition of what is on offer" for a deal at Copenhagen, Denmark, in December.
"Failure to reach broad agreement in Copenhagen would be morally inexcusable, economically short-sighted and politically unwise," he said.
"The science demands it. The world economy needs it."
Hu Jintao, China's president, announced significant carbon emissions cuts for the world's biggest emitter of greenhouse gases.
The Chinese leader said carbon dioxide emissions would be cut by a "notable margin" by 2020 from their 2005 levels.
Obama and Hu are scheduled to meet one-on-one after the summit.
Jose Manuel Barroso, the European Commission president, said talks were "dangerously close to deadlock" and in danger of an "acrimonious collapse", earlier on Tuesday.
But the UN summit and the G20 summit in Pittsburgh later this week seek to add pressure on rich nations to commit to greenhouse gas cuts and to pay for poorer nations to burn less coal and preserve their forests.
Heatwaves, droughts, melting glaciers, loss of the Greenland ice sheet and other dangers are fast approaching, Rajendra Pachauri, chairman of the UN Intergovernmental Panel on Climate Change (IPCC), said earlier this week.
Obama's legislative initiatives to reduce carbon emissions in the US have recently been overshadowed by his push for healthcare reform.
But in his speech on Tuesday, he said that the United States had done more over the past eight months to reduce carbon pollution than at any time in history.
"We understand the gravity of the climate threat," the US president said. "We are determined to act. And we will meet our responsibility to future generations."