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 deal to tackle global warming before the main provisions of the existing Kyoto protocol climate treaty expire at the end of 2012.