TIANJIN: The world's two biggest greenhouse gas emitters have clashed at United Nations talks in China.

A US climate envoy accused China's negotiators of trying to renegotiate last year's global climate agreement, and threatened to pursue alternatives to the UN process. China's top negotiator retaliated by saying the stance of ''a developed country I won't name'' was ''quite unacceptable''.

The US deputy special envoy for climate change, Jonathan Pershing, said the first three days of talks in Tianjin were frustrating because participants were ''relitigating'' old arguments about procedure rather than building on the Copenhagen Accord.

His comment underlines wide differences despite efforts to identify common ground this week so that a partial agreement can be signed at a ministers' meeting in Cancun, Mexico, to start next month.

Mr Pershing was concerned no agreement might be reached in Mexico.