The UN climate chief warned Thursday that a deadlock between the United States and the European Union over pollution cuts threatened to derail talks aimed at launching negotiations for a new global warming pact. "I'm very concerned about the pace of things," said Yvo de Boer, as a two-week UN climate conference entered its final stretch. "If we don't get wording on the future, then the whole house of cards falls to pieces." Washington has refused to accept language in a draft document suggesting that industrialized nations consider cutting emissions by 25 percent to 40 percent by 2020 during upcoming negotiations for a successor to the Kyoto Protocol. The European Union and other governments say the figures are based on sound science and necessary to rein in global warming, but the US, Japan and others argue the inclusion of any numbers would limit the scope of future talks.