The US is refusing to sign off on a flagship global climate fund, as already fraught negotiations intensify ahead of next week’s UN climate summit and carbon prices plummet to an all-time low.

It was already feared that souring economic conditions and next year’s US presidential election would make it difficult for the UN summit – which opens on Monday in Durban, South Africa – to make headway on a new global warming deal before the main provisions of the Kyoto protocol climate treaty expire at the end of 2012.

But officials confirmed yesterday that the US, backed by Saudi Arabia, has still not agreed to adopt a blueprint for the Green Climate Fund . The fund is one of the few measures to emerge from seven years of talks on how countries should share the burden of cutting greenhouse gas emissions, which risk raising global temperatures to dangerous levels.