UN Secretary General Ban Ki-moon has called for urgent action on climate change, saying negotiations on reducing emissions were proceeding too slowly.
He said failure to reach agreement at December's climate talks in Copenhagen would be "morally inexcusable".
He was speaking at a UN meeting attended by about 100 world leaders in New York to revitalise the talks.
Attention is likely to focus on Chinese President Hu Jintao, who is expected to unveil new steps to tackle emissions.
The summit in Copenhagen is aimed at approving a global climate change treaty.
Negotiators are trying to agree on a replacement for the Kyoto Protocol to limit carbon emissions.
Chinese targets
Mr Ban called the meeting an attempt to inject momentum into the deadlocked climate talks.
ANALYSIS
Shirong Chen, BBC China analyst
Change from Beijing is partly a reaction to international criticism as China becomes the world's biggest polluter.
The country's rapid economic growth has created demand for more energy and fuel. There is a growing need for Beijing to provide clear answers on what is being done to deal with the problem.
Image-conscious Chinese officials want to be seen as co-operative internationally and accept that China must become part of the solution to major global issues such as the financial crisis and climate change.
China's climate policy shift
"Your decisions will have momentous consequences," he told the assembled leaders.
"The fate of future generations, and the hopes and livelihoods of billions today, rest, literally, with you," he added.
United States President Barack Obama said Americans understood the gravity of the climate threat and were determined to act, but there was much more work to be done.
"If we are flexible and pragmatic; if we can resolve to work tirelessly in common effort, then we will achieve our common purpose: a world that is safer, cleaner, and healthier than the one we found; and a future that is worthy of our children," he said. read more
