The United States and China clashed on the final day of climate change talks on Saturday, accusing each other of blocking progress ahead of a major summit next month on global warming.
The world's two biggest greenhouse gas polluters sparred throughout the six-day United Nations talks in China, triggering anger from environmentalists who said countries were acting in self-interest and not to save the planet.
US climate envoy Jonathan Pershing warned progress at the UN's annual climate summit in Cancun, Mexico, was in jeopardy because of China's refusal to commit to curbing greenhouse gases.
"We have made some very modest progress. But unfortunately it's been quite limited," Pershing said of the talks in the northern Chinese city of Tianjin.
Delegates from more than 170 countries joined the latest round of long-running UN negotiations aimed at eventually securing a binding global treaty on how to limit and cope with climate change.
This would replace the Kyoto Protocol, which expires at the end of 2012 and aims to keep global warming below the threshold that scientists warn will trigger catastrophic damage to the world's climate system.
World leaders failed to broker such a treaty in Copenhagen last year as developed and developing nations battled over who should carry more of the burden in curbing greenhouse gases, which are blamed for global warming.
Pershing said the biggest problem remained the refusal by China and other developing nations to commit through the UN process to curbing their emissions, and to have those efforts monitored and verified.
"These elements are at the heart of the deal. And the lack of progress on these gives us concern about the prospects for Cancun," he said, insisting this was an element agreed to in Copenhagen.
China, on the other hand, insisted all week that the United States and other rich nations should do much more to curb their emissions, highlighting their historic responsibility for the problem.
China's chief climate negotiator, Su Wei, said the United States was throwing up smokescreens to hide its own inaction.
"It's not fair to criticise if you are not doing anything," he said.
Su earlier referred to a Chinese saying that roughly translates as "a pig looking in a mirror" with reference to the United States and what he said was Washington's refusal to acknowledge its own faults.
The UN's climate chief, Christiana Figueres, said the rift had not derailed the Tianjin talks and that important progress had been made on specific issues.
"I would dare say that this week has got us closer to a structured set of decisions that can be agreed in Cancun," said Figueres, the executive secretary of the UN's Framework Convention on Climate Change.
"This week governments had to address together what was doable in Cancun.... They have actually done that."
She said she was confident a plan by rich nations to give developing countries 30 billion dollars to help them cope with climate change would be finalised at Cancun, helping build trust between the two sides.
"I have said and I will continue to say that fast-track finance is the golden key to Cancun. I am confident that the golden key will be dutifully unlocked," she said.
Greenpeace international climate policy director Wendel Trio criticised the hardline stance of the major players in the talks.
"Governments should look at what they can do for the climate, not what the process can do for them," Trio said.
The talks were to end later Saturday after a final session for delegates to discuss the week's events.
Amid the gridlock, a grassroots movement headed by the 350.org and tcktcktck.org environment groups was gearing up for what they said would be the world's biggest day of climate change action on Sunday.
People in more than 180 countries will plant trees, install solar panels, plant organic vegetables and perform other acts to help the environment during the "Global Work Party".