United Nations: Leaders of some 100 nations met here on Tuesday to breathe new life into deadlocked climate change negotiations, but US President Barack Obama warned not to expect a “perfect" deal.

UN secretary general Ban Ki-moon called the biggest ever summit on climate change some 100 days before a high-stakes conference in Copenhagen, which is meant to seal the framework of a successor to the landmark Kyoto Protocol.

“Failure to reach broad agreement in Copenhagen would be morally inexcusable, economically short-sighted and politically unwise," the UN boss said. Ban pointed to worst-case scenarios of UN scientists, who say the world has only 10 years to reverse the course of climate change that would put at risk entire species and worsen natural disasters.

Changed times: US President Barack Obama at the summit on climate change in the UN General Assembly on Tuesday. AP

But he insisted that developing nations also take action to curb carbon emissions blamed for global warming—and warned that a tough road lay ahead as the world emerges from its worst economic crisis in decades. “All of us will face doubts and difficulties in our own capitals as we try to reach a lasting solution to the climate challenge," Obama said.

“Unease is no excuse for inaction. And we must not allow the perfect to become the enemy of progress...Each of us must do what we can when we can to grow our economies without endangering our planet—and we must all do it together," he said.

Under Obama, the US House of Representatives approved the first ever mandatory national cuts in carbon emissions. But the measure only squeaked through and still awaits Senate action.

Former US president George W. Bush—breaking with the leaders of virtually all other rich nations—refused to take part in the Kyoto Protocol, calling it unfair by not requiring emission cuts by emerging powers.

“Yes, the developed nations that caused much of the damage to our climate over the last century still have a responsibility to lead. And we will continue to do so," Obama said.

“But those rapidly growing developing nations that will produce nearly all the growth in global carbon emissions in the decades ahead must do their part as well," he said.

Obama was expected to receive a response from President Hu Jintao of China, which, according to some measures, has surpassed the US as the top emitter of greenhouse gases. Chinese officials have spent recent days in private talks billing Hu’s address as a major announcement.

UN climate chief Yvo de Boer said he was advised that China would make “an ambitious" statement at the summit. “This policy will make China become the world leader on climate change," de Boer told a small group of reporters.

“I have very high expectations on what President Hu will be announcing," he said, explaining that the measures will “take Chinese emissions very significantly away from where they would have been without a climate policy."

China, along with other developing nations such as India, has long resisted committing to any cuts in greenhouse gases, arguing that rich nations bear historical responsibility.

Japan’s new prime minister, Yukio Hatoyama, set to debut on the world stage at the UN, has said Tokyo will commit to 25% emission cuts from 1990 levels by 2020, against predecessor Taro Aso’s 8% goal.