U.N. climate talks have moved backward rather than moving toward an accord later this year as some nations have balked at pledges to cut greenhouse gas emissions.
The US envoy said some countries have “walked away” from commitments made at Copenhagen last year to limit emissions.
But the top U.N. climate official, Christiana Figueres, said progress had been made towards an eventual deal.
Negotiators are working towards the next climate summit in November in Cancun.
“At this point, I am very concerned,” said chief US negotiator Jonathan Pershing at the conclusion of a week of talks in Bonn.
“Unfortunately, what we have seen over and over this week is that some countries are walking back from progress made in Copenhagen, and what was agreed there.”
Pershing said some of the major developing countries were backing away from commitments to slow the growth of their greenhouse gas emissions, saying such controls should only apply to industrialized countries.
The accord reached at the fractious summit in Copenhagen last year recognized the need to prevent global temperatures from increasing 3.6° F above pre-industrial levels.
In the non-binding agreement, rich countries promised to deliver $100 billion a year by 2020 to help poor countries cope with the impacts of climate change.
“This imbalance is not helpful and could seriously endanger the prospects of securing the successful outcome the world needs from the Cancun climate conference,” said EU climate commissioner Connie Hedegaard.
New climate text under discussion has increased to 34 pages from 17 as new proposals are added or old ones reinserted.
The blueprint contains a set of draft decisions for final U.N. talks in Cancun, in November, including the impact of agriculture on emissions, carbon market mechanisms and the mechanics and impact of moving to a low-emissions future.
A grouping of small island nations – those most affected by rising sea levels – said rich nations’ pledges to cut emissions was not enough.
The leader of the 43-nation Association of Small Island States, Dessima Williams of Grenada, said the lack of legislation in the US to curb emissions was a setback.
“That has been taken as a signal by some that nothing can occur,” she said.
The U.S. Senate dropped efforts to put emissions curbs in an energy bill that is now focused narrowly on reforming offshore drilling, but has said it will stick by its 2020 target for reducing emissions.