BALI, Indonesia — The U.S. and Europe headed toward a compromise today at the U.N. climate conference, breaking a deadlock over how ambitious the goal should be in negotiating future cutbacks in global warming gases, officials said.

Delegates for days had sparred over the wording of the conference's main document, particularly the European Union's suggestion of a goal of reducing emissions by between 25 percent and 40 percent below 1990 levels by 2020.

Trying to break the deadlock, Indonesian Environment Minister Rachmat Witoelar — the conference president — proposed revised language early today dropping those mid-range numbers but still reaffirming that emissions should be reduced at least by half by 2050.

U.N. climate chief Yvo de Boer said a deal was certain. "Absolutely. The only question is how long is it going to take to get, how long we will have to stay up to wait for it," he said.

He told reporters the mid-range 25 percent to 40 percent was implicit — "an inevitable stop on that road" — in the 50 percent goal by 2050.

Witoelar's proposal gave the two sides room to work out the long-expected compromise, producing a relatively vague mandate for two years of negotiations.

"We are sure we are able to reach an agreement," said German Environmental Minister Sigmar Gabriel. "All parties are ambitious to tackle climate change and to have success and the development of the international climate policy."

The annual assembly's main goal was to launch negotiations for a regime of deeper emissions reductions to succeed the Kyoto Protocol, which requires 37 industrial nations to cut output of carbon dioxide and other greenhouse gases by 5 percent below 1990 levels by 2012.

The United States is the only major industrial nation to reject Kyoto.