By Deborah Zabarenko, Environment Correspondent

WASHINGTON, Oct 19 (Reuters) - Global warming talks in Bali in December need to make a breakthrough or international efforts to limit greenhouse gases could be in "deep trouble", the top U.N. climate official said on Friday.

"If we're not on the eve of a breakthrough in Bali, we can spend the next six years waiting for the next report from the Intergovernmental Panel on Climate Change, public interest will slip away and we will be in deep trouble," said Yvo de Boer, who heads the United Nations Framework Convention on Climate Change.

De Boer said the U.N. Bali gathering needed to do four things to start negotiations for a way to curb climate-warming gases after the current accord, the Kyoto Protocol, expires in 2012.

The meeting must agree to launch negotiations, determine the areas of discussion, decide on a deadline and create a mechanism for the negotiations, de Boer said, .

"(The Bali meeting) doesn't have to deliver the perfect climate change regime, it doesn't have to answer all the questions, it doesn't have to solve all the problems, it just has to get the process going," de Boer said at a World Bank forum.

The Bali meeting follows three reports this year by the U.N. Intergovernmental Panel on Climate Change, which shared the Nobel Peace Prize last week with former U.S. Vice President Al Gore.

The first said with 90 percent probability that global warming is real, that human activities cause it and that the problem is urgent. The second detailed the potentially disastrous consequences of unchecked climate change and the third focused on what to do about it.

The United States will attend the Bali meeting, but the Bush administration has consistently rejected setting mandatory targets for reducing emissions of carbon -- a powerful greenhouse gas -- in favor of voluntary measures.

Valli Moosa, president of the World Conservation Union, said this stance hindered agreement with the Group of 77 developing countries.

"A big obstacle to making progress even within the G77 is the resistance of the United States to multilateral environmental agreement," Moosa said at the forum.

"Let's not hold up some kind of irrational hope that the G77 is going to move quickly towards some sort of differentiated approach, some new setting of targets ... without meaningful movement of the United States in becoming part of a global regime," Moosa said.

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