Progress towards a global treaty to fight climate change took an important step forward today when the US said it would not demand that China commits to binding cuts of its greenhouse gas emissions.

The move came on the last day of the latest round of UN climate change talks involving 183 nations, which aim to produce a deal in Copenhagen in December.

Jonathan Pershing, head of the US delegation in Bonn, said developing nations – seeking to grow their economies and alleviate poverty – would instead be asked to commit to other actions. These include boosting energy efficiency standards and improving the take-up of renewable energy, but would not deliver specific reductions. He said: "We're saying that the actions of developing countries should be binding, not the outcomes of those actions."

Only developed nations, including the US, would be expected to guarantee cuts. The pledge was included in a US blueprint for a climate change deal submitted to the Bonn meeting, which Pershing said was based on the need for the rich nations to cut greenhouse gases 80% by 2050.

The US plan, if approved, could replace the existing Kyoto protocol. The lack of any carbon targets for developing nations in the Kyoto protocol was the reason the US never ratified that treaty. While such cuts were believed to be unrealistic even in the new treaty, the first clear acceptance of that at the UN talks by the US is being seen as significant. EU officials said they were studying the US proposal.

China and the US are the two biggest polluters in the world, making their positions on the deal critical. In a separate submission to the talks, China was among a group of developing nations that called on rich countries including Britain and the US to cut emissions by 2020 by 40% on 1990 levels. According to environmental group WWF, commitments made by developing countries so far add up only to about a 10% cut — Japan this week proposed an effective 8% cut in its emissions.

Observers see the 40% demand as unrealistic, suggesting the US move amounts to blinking first in the negotiations. But back channel negotiations, revealed by the Guardian last month, showed the two nations are searching for a deal.

John Ashe, who chaired discussions at Bonn on how Kyoto targets could be extended, said many of the targets put forward could yet be revised as the Copenhagen deadline loomed. "There is always an initial move and then a final move. I don't believe we're in the final stage yet."

He said China should agree to take actions to control emissions that were measured and reported to the international community.

In Washington, Todd Stern, the state department's climate change envoy, said the US still expected China to take serious moves towards a cleaner economy. "We are expecting China to reduce their emissions very considerably compared to where they would otherwise be [with] a business as usual trajectory."

At the end of the talks, the UN's top climate official said significant progess had been made. Yvo de Boer, executive secretary of the UN Framework Convention on Climate Change, said: "A big achievement of this meeting is that governments have made it clearer what they want to see in the Copenhagen agreed outcome."

But green campaigners criticised the failure to resolve any major issues, such as an overall target for 2020 emission reductions or concrete proposals on funding for poor nations to deal with global warming.

Antonio Hill of Oxfam said: "The countries that created the nightmare are refusing to lift a finger to prevent it becoming a reality. Rich country delegates have spent two weeks talking but have done nothing on the issues that really matter. They may be kidding themselves they are working towards a deal but they are not kidding anyone else."