NEW YORK - The US and China committed to tackling their greenhouse-gas emissions blamed for global warming and asked each other to do more to halt the rise in global temperatures, at the start of a one-day UN summit on climate change

Tuesday.

US President Barack Obama acknowledged his country has been slow to respond to the threat of climate change, but said: “This is a new day.”

“We understand the gravity of the climate threat. We are determined to act,” Obama said, noting efforts by his own administration to reduce vehicle emissions and promote renewable energy.

Obama has come under pressure from other countries as major legislation to curb US emissions has stalled in the Senate. But Obama said it was up to all major polluters to take action.

He called on other countries - especially emerging economies like China and India - to make the commitments necessary to lower their own growing emissions, in order to reach agreement on a new global deal by a crucial UN summit in Copenhagen in December.

“We cannot meet this challenge unless all the largest emitters of greenhouse-gas pollution act together. There is no other way,” Obama said.

Chinese President Hu Jintao said his country was determined to lower its burgeoning levels of greenhouse-gas emissions, but tasked richer nations with helping the poor adapt without destroying their economies.

“China has taken and will continue to take determined and practical steps to tackle this challenge,” said Hu, who is the first Chinese leader in 30 years to join the annual opening of the UN General Assembly.

“We should combine our efforts to address climate change with those to promote the growth of developing countries,” he said.

How much wealthy countries should give developing nations to lower their own emissions has been a key disagreement as government negotiators try to hammer out a new global climate treaty.

China has proposed that wealthy nations dedicate 1 percent of their gross domestic product to helping developing countries tackle global warming. 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.