(Corrects paragraph 5 to ... beyond 2012 ..., not beyond 2009)
(Recasts, adds reaction, details throughout)
By Emma Graham-Harrison
NUSA DUA, Indonesia, Dec 15 (Reuters) - Nearly 200 nations agreed at U.N.-led talks in Bali on Saturday to launch negotiations on a new pact to fight global warming after a reversal by the United States allowed a historic breakthrough.
Washington said the agreement marked a new chapter in climate diplomacy after six years of disputes with major allies since President George W. Bush pulled out in 2001 from the Kyoto Protocol, the main existing plan for combating warming.
"This is the defining moment for me and my mandate as secretary-general," U.N. Secretary-General Ban Ki-moon said after making a special return trip to Bali to implore delegates to overcome deadlock after the talks ran a day into overtime.
Ban had been on a visit to East Timor. "I am deeply grateful to many member states for their spirit of flexibility and compromise," Ban told Reuters.
The Bali meeting approved a "roadmap" for two years of talks to adopt a new treaty to succeed Kyoto beyond 2012, widening it to the United States and developing nations such as China and India. Under the deal, a successor pact will be agreed at a meeting in Copenhagen in late 2009.
The deal after two weeks of talks came after the United States dramatically dropped opposition to a proposal by the main developing-nation bloc, the G77, for rich nations to do more to help the developing world fight rising greenhouse emissions.
Indonesian Environment Minister Rachmat Witoelar, the host of the talks, banged down the gavel on the deal to rapturous applause from delegates, weary after intensive talks and numerous disputes over the past fortnight.
"I think it was encouraging. That was a real sign of willingness to compromise," Yvo de Boer, head of the U.N. Climate Change Secretariat, said of the U.S. climbdown.
The accord marks a step towards slowing global warming that the U.N. climate panel says is caused by human activities led by burning fossil fuels that produce carbon dioxide, the main greenhouse gas.
Scientists say rising temperatures could cause seas to rise sharply, glaciers to melt, storms and droughts to become more intense and mass migration of climate refugees.
"The U.S. has been humbled by the overwhelming message by developing countries that they are ready to be engaged with the problem, and it's been humiliated by the world community. I've never seen such a flip-flop in an environmental treaty context ever," said Bill Hare of Greenpeace.
The European Union was pleased with the deal.
"It was exactly what we wanted. We are indeed very pleased," said Humberto Rosa, head of the European Union delegation.
"We will have now two tremendously demanding years, starting right in January. Many meetings, many discussions, many people passing many hours doing things," he said.
Agreement on a pact in 2009 would give governments time to ratify the pact and give certainty to markets and investors wanting to switch to cleaner energy technologies, such as wind turbines and solar panels.
Kyoto binds all industrial countries except the United States to cut emissions of greenhouse gases between 2008 and 2012. Developing nations are exempt and the new negotiations will seek to bind all countries to emission curbs from 2013.
DAY OF DRAMA
In a day of drama and emotional speeches, nations had berated and booed the American representatives for holding out. A wave of relief swept the room when the United States relented.
"The United States is very committed to this effort and just wants to really ensure we all act together," said Paula Dobriansky, head of the U.S. delegation.
"With that, Mr Chairman, let me say to you we will go forward and join consensus," she said to cheers and claps from delegates. The United States is the leading greenhouse gas emitter, ahead of China, Russia and India.
"There is no question that we have opened a new page and are moving forward together. It is a strong commiment jointly reached by all countries to advance negotiations," said James Connaughton, Chairman of the White House Council on Environmental Quality, in Bali.
"This is not a step taken alone by America. This is a step taken by all the countries that the time had come to open a new chapter," he added. -- For Reuters latest environment blogs click on: blogs.reuters.com/environment/ (Reporting by Adhityani Arga, Sugita Katyal, Alister Doyle, Ed Davies, Gde Anugrah Arka and Gerard Wynn; writing by David Fogarty; editing by Alister Doyle)