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The U.S. announced late Friday that it had reached a "meaningful agreement" with China, India, South Africa and Brazil over a climate change plan. However, as U.S. President Barack Obama trumpeted the deal as "unprecedented," other leaders questioned its veracity.

The eleventh-hour framework for fighting global warming, known as the Copenhagen Accord, would ensure that rich countries would help out poor ones in bringing down carbon emissions.

"Today we've made (a) meaningful and unprecedented breakthrough in Copenhagen," said Obama on Friday evening.

He added that the deal, which focuses on transparency, mitigation and financing, will usher in a "new era of international action."

"We have come a long way but we have much further to go," he said.

Obama said that the deal was reached at the end of the conference, yet some leaders were still meeting at a plenary session late Friday. Many specifics over the plan's mechanics were also vague.

Meanwhile, poorer nations assailed the plan because it lacks any binding legal requirements and does not set strict reduction targets.

Still, Obama's last-minute plan resulted in a three-page document that pledges US$30 billion over the next three years for developing nations. The plan also sketched out a goal of pumping US$100 billion each year into the fund by 2020.

The plan also sets out to limit global warming to 2 C, which is stronger than previous declarations.

There had been several stumbling blocks in the talks, including concern over China's involvement, and leaders such as Prime Minister Stephen Harper were prepared to stay in Copenhagen longer than expected.

Earlier Friday, Obama arrived in Copenhagen, where he attended an emergency meeting with 19 other leaders. Obama arrived to drive a deal, which had eluded leaders throughout the week.

Australian Prime Minister Kevin Rudd, British Prime Minister Gordon Brown, German Chancellor Angela Merkel and French President Nicolas Sarkozy were among the attendees. Chinese Premier Wen Jiabao chose not to attend the high-level meeting, sending an envoy in his place.

Harper was not at the meeting as he was not on the guest list. A spokesperson for the prime minister said the list of countries invited to the meeting had been compiled for Obama's people as a representative mix of regions and carbon emitters.

Following the meeting, Harper had a brief audience with Obama over lunch.

The prime minister has kept a low profile in Copenhagen since arriving earlier this week. In his first public comments at the summit, he held a press conference on Friday evening and endorsed the agreement.

"There's much more work to be done but I think the toughest hurdles were cleared here today," Harper said. "It is a good agreement that achieves Canada's objectives."

On Thursday night, Environment Minister Jim Prentice addressed the UN climate talks instead of Harper, while the prime minister attended a royal dinner hosted by the Queen of Denmark.

Also Friday, Canada was stamped with the "Colossal Fossil" designation, a dubious honour given to the country by environment groups which rank its greenhouse gas target as being "among the worst in the world."

Divisions in Copenhagen

The talks in Copenhagen had been marred by a growing division between rich and poor countries, as well as between the United States and China -- the world's two biggest polluters. Delegates blamed the rival countries for the lack of a political agreement all nations were expected to sign earlier in the day.

The Chinese and U.S. leaders had also failed to announce any new commitments on slashing their greenhouse gas emissions.

Wen and Obama met privately on Friday, as world leaders pressed to salvage the summit.

Abandoning hope of a comprehensive deal, a group of about 25 countries tried to agree on a two-page political statement that would lay out critical elements. It called for spending $30 billion in the next three years to help poor countries cope with climate change, spending that would ramp up to $100 billion per year by 2020.

As talks continued, new drafts of the same document added clauses that evolved during the day. Later drafts said rich countries should cut their emissions by 80 per cent by the year 2050.

A separate group of leaders worked on a potential deal involving emission cuts that UN Environment Program Director Achim Steiner said was "doable."

But James Hansen, a professor at Columbia University who is credited with bringing global warming to the attention of U.S. Congress in the 1980s, said he doubts that measures being discussed at Copenhagen, such a cap-and-trade system, would actually force countries to produce fewer greenhouse gases.

"Emissions which had been going up 1.5 per cent per year accelerated after the Kyoto protocol to 3 per cent per year -- and I think the same thing is going to happen now," he told Â鶹ӰÊÓ Channel.

"We're not going to in fact turn down the emissions with the kind of actions that are being talked about."

"As long as fossil fuels are the cheapest energy, then they're going to continue to be used, and even more so. So you have to put a price on the carbon, on the fossil fuels -- coal, oil and gas."

With files from The Canadian Press and The Associated Press