The world's two biggest carbon emitters clashed at UN climate talks in China today as the United States' top climate envoy accused his counterparts of trying to renegotiate last year's global climate agreement, and threatened to pursue alternatives to the United Nations negotiation track. China retaliated by calling the US's overall negotiating stance "totally unacceptable."

Jonathan Pershing, the US deputy special envoy for climate change, said the first three days of talks in Tianjin had yielded disappointing results because participants were revisiting old arguments over procedure rather than building on the Copenhagen accord.

"What is frustrating in these negotiations is to see countries not using that as the basis, but relitigating things that we more resolved over the course of the Copenhagen negations," he said.

His comments underline the wide differences between nations despite efforts to try to identify common ground this week so that a partial agreement can be signed at a ministerial level meeting in Cancún later this year.

Given the slow rate of progress, Pershing said there was a concern that no agreement would be possible in Mexico. Echoing comments made this week by EU negotiators, he said it could damage the UN system. "It something to be considered seriously, because the process is going to be very hard-pressed to continue to meet and to continue to have these enormous sessions with a lot of people travelling to them unless we can use the process to good effect," he said. "It may mean that we don't use this process exclusively as the way to move forward."

While there is no suggestion of a full withdrawal from the UN process, the US appears to have hardened its position since Copenhagen amid rising domestic political pressure and the absence of climate legislation.

China has responded in kind. Dropping the diplomatic language that characterised public statements on the first two days, Xie Zhenhua, the head of the host's negotiating team, made little attempt to conceal the target of his frustration.

"A developed country I won't name hasn't done a job for itself. It has not provided financing or technology to other countries, yet it asks them to accept stringent monitoring and voluntary domestic actions," Xie told reporters. "It's totally outrageous. It's quite unacceptable."

There were other signs of rising stress at the halfway point, when the workmanlike calm of the first three days gave way to heated exchanges during a stock-taking session.

When the chairman of the session drew up a to-do list for an agreement at Cancun, the proposal was denounced by China and other developing nations as "premature and imbalanced."

Progress was registered on the issues of forestry, technology transfer and financing for poor nations to cope with climate change, but discussions on the key topic of emission reduction targets were blocked by developing nations.

European officials described the tactics as inexplicable. "We are losing a tremendously important issue," said Jürgen Lefevere, climate strategy adviser to the European commission. "The Cancún target should be to anchor the targets pledged so far, to get them on a paper with a UN heading."

Poorer nations are reluctant to have their pledges mixed with those of richer nations, which have a greater historical responsibility for climate change, particularly given the political uncertainty surrounding the ability of the US to achieve its goals.

Without locking the existing commitments in place, EU officials say it will be difficult to move the discussion forward to the more ambitious goals needed to achieve the Copenhagen target of keeping global warming within 2C by 2050.

The need for greater action was highlighted in a new report published today by WWF, which showed that even if every country lived up to its Copenhagen pledges by 2020, global emissions would be at least 20% higher than the 40 gigatonne budget needed to avoid dangerous climate change.

"It's clear that some countries are facing up to the necessary transformations of their economies but other countries have failed to endorse this new trend speedily and are risking the safety and prosperity of all," said Gordon Shepherd, leader of WWF's global climate initiative.