The measure has been rejected by the United States, which wants emerging economies like China to commit to hard targets for lowering their own emissions.

China said his country would work to slow the growth of its emissions by a 'notable margin' by 2020. The Asian powerhouse would also increase its share of renewable energy use to 15 percent of the total by that year.

'China stands ready to join hands with all countries to build an even better future for the generations to come', Hu said.

Earlier, UN Secretary General Ban Ki-moon warned that the worst consequences of climate change were drawing ever closer as he urged world leaders to thrash out an ambitious new global treaty by December.

'Now is your moment to act,' Ban told more than 100 leaders who gathered to discuss curbs to halt global warming. 'The fate of future generations, and the hopes and livelihoods of billions today rest, literally, with you.'

Ban, who has made tackling climate change his top priority during his time in office, is hoping Tuesday's gathering will inject the political will needed to complete the talks in Copenhagen for a successor to the Kyoto Protocol, the world's first climate treaty, which expires in 2012.

Obama and Hu, who are both debuting on the UN stage, were under close scrutiny for their views on a topic on which scientists and politicians still hotly disagree. The US and China are by far the world's biggest emitters of carbon dioxide, which contributes to the rise in global temperatures.

The summit is billed by the UN as the largest climate summit in history, as 86 presidents and 36 prime ministers will debate climate change in a series of roundtables throughout the day. Ban will summarize and publicize their views at the end of the gathering.

The climate summit comes ahead of Wednesday's annual opening of the UN General Assembly, which brings together leaders from around the world for 10 days of speeches about global issues.