COPENHAGEN: World powers expressed mounting concern on Thursday that the UN climate summit was heading for failure and an agreement may have to wait until next year.

The United States sought concessions from emerging economic giants on emissions, while the European Union expressed "concern" at the lack of progress ahead of a gathering of more than 120 world leaders on Friday.

US Secretary of State Hillary Clinton, who announced that Washington would contribute to a 100-billion-dollar fund to held poor nations cope with climate change, said it was "time to take an historic step we can all be proud of."

Speaking ahead of the arrival of President Barack Obama, she accused developing nations -- without naming them -- of backsliding on pledges to open their emissions controls to scrutiny.

China and India say they are willing to take voluntary measures to slow forecast surges in emissions. But they are reluctant to accept tight international scrutiny and insist that rich nations should bear the brunt of the substantial reduction targets.

"There have been occasions in this past year when all the major economies have committed to transparency," Clinton told a press conference.

"Now that we are trying to define what transparency means and how we would both implement it and observe it, there's a backing away from transparency -- and that to us is something that undermines the whole effort that we're engaged in," she said.

"If there is not even a commitment to pursue transparency, that's kind of a deal breaker for us."

Danish Prime Minister Lars Loekke Rasmussen, the conference chairman, declared the meeting was "now at a critical juncture and we have now agreed on how to proceed."

"Now we rely on the willingness of all parties to take that extra step that would enable us to make the deal that is expected of us," he said.

The European Union called for urgent efforts to save the Copenhagen summit.

"The European Union is concerned by the lack of progress in the negotiations. We encourage all parties to urgently go to the outer limits of their flexibility so that talks can move forward," the EU Commission and the Swedish EU presidency said in a joint statement.

"The European Union has put concrete proposals on the table and maintains its conditional offer to do more, if others, especially the major emitters, improve their offers as well," the statement added.

Australian Prime Minister Kevin Rudd said in a speech to conference: "I fear a triumph of form over substance. I fear a triumph of inaction over action."

German Chancellor Angela Merkel said the events in Copenhagen were "not good" and the summit would be a failure if nations did not agree to cap global warming at two degrees Celsius (3.6 Fahrenheit) compared to pre-industrial levels.

"At this hour, I don't know if that will be successful," she said in Berlin.

Tuvalu, a Pacific archipelago which is one of the countries most at risk from rising sea levels, said it would only agree to a 1.5 C rise but complained that it had been kept out of key talks.

"We will leave this meeting with a bitter taste in our mouth. The true victims of climate change have not been heard here," Prime Minister Apisai Ielemia told AFP.

A deeply gloomy senior delegate told AFP: "It won't be feasible to get a complete agreement unless it's just one page. We need several more months."

Scientists say the cost of failure on limiting the rise in temperatures will be catastrophic with hundreds of millions of people already facing worsening drought, flood, storms and rising seas.

The United States was widely condemned for foot dragging on climate change under President George W. Bush, and Obama is hoping that his presence will be evidence of a transformation of policy.

In a sign of goodwill, Clinton announced the US would contribute towards a fund worth 100 billion dollars a year by 2020 to help poor countries cope with climate change.

Japan on Wednesday promised to stump up 1.75 trillion yen (19.5 billion dollars) to developing nations if a comprehensive deal is reached at Copenhagen.

The United States and Japan were also among a group of six nations that said they would set up a fund to fight the loss of forests, a leading source of the rising temperatures.