US President Barack Obama has expressed confidence a climate deal can be clinched as dozens of world leaders gather on Wednesday to try to break a deadlock at UN climate talks.

“The president believes that we can get an operational agreement that makes sense in Copenhagen,” White House spokesman Robert Gibbs told a briefing in Washington on Tuesday, three days before a deadline on a new UN deal to combat climate change.

Leaders including Venezuelan President Hugo Chavez, Zimbabwe's President Robert Mugabe and British Prime Minister Gordon Brown were set to give speeches at the December 7-18 climate meeting, until now dominated by environment ministers.

The world leaders have until a main summit on Friday to agree a deal under a deadline set at a meeting in Bali, Indonesia, in 2007. Negotiations since Bali have been marred by mistrust between rich and poor nations.

US Secretary of State Hillary Clinton wrote in an International Herald Tribune opinion piece on Tuesday that success in Copenhagen demanded that all major economies take decisive action and agree to a system that is transparent and trusted.

“The president believes that to get an agreement that is truly operational, that we have to have that – that transparency.  That's one of the things that he'll work on as we go forward,” Gibbs said.

As the deadline approaches for a pact that would favour a shift to low-carbon businesses, some politicians are warning of the risks of failure in the 193-nation negotiations, even as they urge compromises to allow a breakthrough.

“It's possible that we will not reach agreement and it's also true that there are many issues to be sorted out,” Brown said in Copenhagen on Tuesday night.

“In these very hours we are balancing between success and failure,” said Danish President of the two-week meeting, Connie Hedegaard, at the opening of a high-level phase of the talks on Tuesday night.

A formal summit of more than 120 world leaders on Thursday and Friday is due to agree a global deal to slow rising temperatures set to cause heat waves, floods, desertification and rising ocean levels.

Environment ministers have been meeting since the weekend, trying to ease splits between rich and poor nations about sharing out the burden of curbs in emissions of greenhouse gases and raising billions of dollars in new funds to help the poor.

Deeper cuts in greenhouse gases

“The absolute core benchmark for success is for the first time in history to have an agreement between rich and poor countries on this common challenge,” Australian Prime Minister Kevin Rudd said in Copenhagen.

The United Nations wants developed nations to cut their greenhouse gas emissions more deeply than planned by 2020, wants developing countries to do more to slow their rising emissions and wants billions of dollars in aid to help the poor.

China, the United States, Russia and India are the top emitters and have all set goals for curbing emissions in recent months. But rich and poor nations are demanding more than the other side is willing to give.

A major hurdle is that the United States has not yet passed legislation capping its emissions – unlike all its main industrial allies.

Friends of the Earth said that South African Nobel Peace Prize winner Archbishop Desmond Tutu wrote to all African leaders urging them to insist on a deal to limit global warming to a temperature rise of 1.5 Celsius over pre-industrial times.

Many nations favour an easier 2.0 Celsius limit.

“A global goal of about 2 Celsius is to condemn Africa to incineration and no modern development,” according to a copy of the letter. Tutu said that it would be better “to have no deal than to have a bad deal”.

But Brown said the costs of failure to rein in greenhouse gases, mainly from burning fossil fuels, could be huge.

Inaction would cause “a reduction in our national income of up to 20 percent, an economic catastrophe equivalent in this century to the impact of two world wars and the Great Depression in the last,” he said in a statement on arrival in Copenhagen.

Major US businesses including Duke Energy, Microsoft and Dow Chemical called for tough US emissions cuts which would mobilise a shift to a greener economy.