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China will increase efforts to improve energy efficiency and cut CO2 emissions, President Hu Jintao has told a UN climate change summit in New York.
Mr Hu said CO2 emissions would be cut by a "notable margin" by 2020, but gave no overall figure.
About 100 leaders are attending the talks, ahead of the Copenhagen summit which is due to approve a new treaty.
UN Secretary General Ban Ki-moon said failure to agree a treaty in December would be "morally inexcusable".
Negotiators for the Copenhagen summit are trying to agree on a replacement for the Kyoto Protocol to limit carbon emissions.
'Momentous consequences'
Mr Ban called the meeting an attempt to inject momentum into the deadlocked climate talks.
"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.
The Chinese president said his country's cuts would be measured by unit of Gross Domestic Product.
He also pledged to "vigorously develop" renewable and nuclear energy.
He restated China's position that developed nations needed to do more than developing nations to fight climate change because they were historically responsible for the problem.
"Developed countries should fulfill the task of emission reduction set in the Kyoto Protocol, continue to undertake substantial mid-term quantified emission reduction targets and support developing countries in countering climate change," he said.
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.
According to the BBC's UN correspondent, Barbara Plett, discussions have stalled because rich nations are not pledging to cut enough carbon to take the world out of danger, while poorer countries are refusing to commit to binding caps, saying this would prevent them from developing their economies.
China's role is crucial, because it is both an emerging economy and a big polluter, our correspondent says.
Despite all its advances in green technology, China still gets 70% of its energy from coal - and as its economy increases, this means yet more growth in greenhouse gases, our correspondent says.
Pressure on US
There is also concern about the world's other big polluter, the United States.
President Obama has recognised climate change as a pressing issue, unlike the previous administration, our UN correspondent says.
He has already announced a target of returning to 1990 levels of greenhouse emissions by 2020, but critics say Washington is moving too slowly on legislation which does not go far enough.
President Obama is currently dogged by domestic issues such as the economy and healthcare reforms, but his speech to the UN meeting will still be watched for signs he is willing to fulfil his pledge to take the lead in reaching a global carbon deal.
A demonstration of political will by both China and the US will be important in breaking the deadlock in negotiations, correspondents say.
China and the US each account for about 20% of the world's greenhouse gas pollution from coal, natural gas and oil.
The European Union is responsible for 14%, followed by Russia and India with 5% each.
Source: BBC