The United States will not agree to targets cutting greenhouse gas emissions unless developing countries, particularly China, make similar moves, US climate envoy Todd Stern warned on Wednesday.
"No country holds the fate of the Earth in its hands more than China," Stern told the House Foreign Affairs Committee, weeks before a major climate change summit in Copenhagen.
Stern said new climate rules could include exemptions for developing countries to ensure that growth is not hampered but emerging giants like China, India and Brazil should pull their weight.
"What we do not agree with, though, is that we should commit to implement what we promise to do, while major developing countries make no commitment at all," he said.
His comments come as divisions between developed and developing countries threaten to scupper a Copenhagen climate deal to replace the Kyoto Protocol.
"We have 32 days left before the beginning of the Copenhagen conference and there is still a lot of work to do," Stern said.
"It's fair to say that the progress has been too slow, especially in the formal UN negotiating track," he stressed.
"The developed-developing country divide that has run down the centre of climate change discussions for the past 17 years is still I'm afraid alive and well.
But Stern said the situation was not all gloomy.
"Paradoxically, while the negotiations are in a difficult state, it's also true that we are at a moment in history when more countries, including China, Brazil and South Africa are taking stronger actions or are poised to take stronger actions than ever before to combat climate change."
He addressed members of Congress as they debate a bill aimed at reducing greenhouse gas emissions in the United States, which many see as a prerequisite to a deal at Copenhagen.
Climate talks in Barcelona resumed on Wednesday after an angry spat but negotiators admitted chances for sealing a hoped-for UN treaty on global warming by year's end were vanishing.
On Tuesday, African countries boycotted the Barcelona climate talks.
The bloc of 50 nations accused rich counterparts of backsliding on promises to curb man-made carbon emissions blamed for global warming, demanding they slash their pollution by at least 40 per cent by 2020 over 1990 levels.
The squabble blocked talks among countries that have ratified the Kyoto Protocol, the cornerstone pact of the UN Framework Convention on Climate Change (UNFCCC).
The twin-track process was launched in Bali in 2007 with the goal of concluding a post-2012 treaty among the UNFCCC's 192 parties at a December 7-18 showdown in Copenhagen.