COPENHAGEN,  (Reuters) – U.S. President Barack Obama  has expressed confidence a climate deal can be clinched as  dozens of world leaders gather today to try to break a  deadlock at U.N. 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 U.N. 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 Dec. 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.

U.S. Secretary of State Hillary Clinton wrote in an  International Herald Tribune opinion piece yesterday 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 last 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 last night.

A formal summit of more than 120 world leaders tomorrow  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 INGREENHOUSE 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”