The United States denied on Thursday it was trying to delay a new global climate deal until 2020, saying it supported an EU proposal that aims to chart a path to a more ambitious pact to fight climate change.
Delegates from almost 200 countries have until Friday to decide whether to commit to signing up to an internationally binding climate deal by 2015 at the latest.
Some countries and pressure groups say the United States is trying to delay the start of a legally binding deal until after 2020, because of deep splits between Democrats and Republicans in the U.S. Congress and because environmental curbs are seen as a vote loser ahead of U.S. presidential elections next year.
"It is completely off base to suggest the U.S. is proposing it will delay action to 2020," U.S. climate envoy Todd Stern told reporters.
"The EU has called for a roadmap (to a future deal). We support that," he said.
The European Union is pushing to complete talks for a global deal that would bind all major polluters to cut emissions by 2015. But even if that were agreed, such a pact would likely only come into force five years later.
The United States said earlier this week it supported a discussion that would lead to an emission cut deal, even one that was legally binding, but would not commit itself to set dates or a set outcome.
The two biggest issues for negotiators in Durban are finding a way of updating the Kyoto Protocol, the only global pact that enforces carbon cuts, and finding the necessary cash to help poor countries tackle climate change.
Stern's apparent support for the EU roadmap met with some scepticism, particularly from the island nations most threatened by the rising sea levels caused by global warming.
"Thank you very much, let me see that in the negotiation room, let me see that in the text," Grenada's Foreign Minister Karl Hood said in response.
The United States has set a voluntary target of cutting its emissions by 17 percent below 2005 levels by 2020, equivalent to three percent below 1990 levels. The EU goal is a 20 percent reduction by 2020 compared to 1990 levels.
The United States has said it will only make its emissions cuts binding under an international agreement if China and other developing countries that are big polluters back their commitments with equal legal force.
U.S. delegate heckled
Only modest steps towards a broader deal on cutting greenhouse gases remain the most likely outcome of the Durban talks, which officially end on Friday.
Change at the current pace will not be enough to save the Maldives -- a low-lying island chain popular with Western tourists that risks disappearing under rising sea levels caused by global warming.
"One of the reasons why the U.S. is reluctant to talk about anything before 2020 is because it will affect its presidential elections," the Maldives' Environment Minister Mohamed Aslam told Reuters in an interview.
"They are being foolish but you can't just blame the government. It's also down to its citizens. The government is just saying 'our people do not want this'."
Public concern about climate change in the United States has been steadily falling since 2007, an opinion poll showed in August, as people focus on more immediate economic concerns and question the science behind climate change.
But an American student was thrown out of Thursday's talks after she gained entrance to a high-level negotiating session and accused the U.S. Congress of obstructing progress towards a new global deal.
"Time is running out"
Poor nations say they are already paying the price of climate change and want action now.
Environment Minister Herman Rosa Chavez, said a tropical depression two months ago flooded 10 percent of El Salvador and caused losses of $840 million -- four percent of gross domestic product.
"We went from being impacted by one extreme weather event per decade in the 1960s and 1970s, to nine in the last 10 years," he said.
French Ecology Minister Nathalie Kosciusko-Morizet said private partnerships and voluntary agreements were no substitute for a comprehensive, legally binding agreement including all major economies.
"There's no other way. There's no Plan B for the planet," she told delegates. "Time is running out. The remaining hours of the talks here in Durban are crucial."
(Additional reporting by Jon Herskovitz, Andrew Allan and Agnieszka Flak; Editing by Jon Boyle)