But the chief U.S. delegate urged negotiators from 175 countries to think of longer-term objectives rather than focus on short-range targets that would be difficult to meet.
"We are actively working to move forward aggressively," Jonathan Pershing told hundreds of delegates on the final day of talks before the negotiations adjourn for two months.
The U.S. favors adopting "pathways" toward a mid-century goal rather than a definitive near-term target, Pershing said.
The two visions, while not mutually exclusive, will require dexterity by negotiators seeking to conclude a new global warming accord by December to replace the Kyoto Protocol, the 1998 agreement the United States rejected.
Negotiators have informally accepted the conclusion of a panel of U.N. scientists that industrial countries must together cut carbon emissions by 25 to 40 percent from 1990 levels by 2020 to avoid a catastrophic rise in sea levels, changes in weather patterns and disruptions of agriculture that would affect millions of predominantly poor people.
"The numbers being discussed so far are still a significant distance from that range," De Boer told reporters. "More ambition is clearly needed on the part of industrialized countries."
Developing countries also must dramatically slow the growth of their emissions, the scientists of the Intergovernmental Panel on Climate Change said in an exhaustive 2007 report.
De Boer singled out China, India, Mexico, Brazil and South Africa for adopting strategies to slow the growth of their emissions, and said it was important politically that their actions be recognized, "which it is not, at the moment."
The U.S. position, seeking to soften the recommended 2020 target, reflects the difficulty America will have in quickly reducing its greenhouse gases, which the previous Bush administration refused to regulate.
"There is a basis for negotiation," said Kim Carstensen of the World Wildlife Fund for Nature. He said the U.S. stand raises problems for industrial countries, especially in Europe, that have taken action to curb emissions, and for developing countries that are looking for strong action from the world's second largest carbon emitter.
Obama has outlined plans to bring U.S. emissions back to 1990 levels by 2020, a reduction of about 17 percent from the current rate. He wants Congress to pass a cap-and-trade program that would need time to take hold, but which he said would help reduce emissions by 80 percent by 2050.
De Boer said the U.S. promise to fully engage in the negotiations has lifted expectations, and some gaps had been narrowed at the Bonn talks, mainly on technical questions.
But no headway was made on the key issues of adopting emission reduction targets or on how to raise and distribute the $100 billion needed per year to help poor countries adapt to climate change.
"We need more progress now on who will act, how and when," De Boer said.
Those issues could be helped by the meetings of major economies, a forum of 17 countries initiated by President Barack Obama that is due to have its first meeting later this month.
Feeling the pressure of time, delegates were discussing whether to schedule more negotiating sessions in addition to the three others due this year.