THE refusal of the United States to negotiate legally binding cuts to carbon emissions risks derailing the UN climate change summit, environmental groups have warned in a letter to Hillary Clinton.  The letter, from 16 organisations to the US Secretary of State, said President Barack Obama had broken the pledge he made in November 2008 to "engage vigorously in these negotiations and help lead the world towards a new era of global co-operation on climate change".  Instead, the letter claimed, the US is being seen as a "major obstacle" to progress. Signatories included Greenpeace USa, the Natural Resources Defence Council, Oxfam america and the World Wildlife Fund.  after it was made public, the EU delegation at the summit criticised the US for "overlooking the facts" of climate change and suggested it was not doing enough to cut carbon emissions. The EU bloc wants the current talks to agree on a timetable for all major polluters to sign a legally binding agreement to cut their emissions by 2015.  at the opening of the summit in Durban on Monday, the US said it would not consider the EU's proposals until major developing countries such as China and India sign up. China, the world's biggest emitter, is against the 2015 deadline.  Jonathan Pershing, the US deputy special envoy for climate change, added that he believed voluntary cuts agreed by countries at the previous summit in Cancun, Mexico were sufficient until 2020.  However, artur Runge-Metzger, the EU's chief negotiator, said he was "overlooking the facts". "The message from the science is crystal clear: we have to act now," he added.  Mr Runge-Metzger agreed with the letter writers that a refusal by the US to negotiate could stall the summit. "It will always have an effect," he said. "The bigger countries, the major economies, will always look towards the US."