Athens - The newly appointed US ambassador to the UN, Mr Richard Holbrooke, was booed during a speech in Greece when he said Washington backed Turkey's future membership in the European Union."The US supports Cyprus' EU accession, but the US also supports Turkey's eventual membership in the EU," he said in his speech at a graduation ceremony of an Athens-based American college.Earlier in Belgrade the Serbian information minister said Mr Holbrooke made a "terrible diplomatic gaffe" by meeting Kosovo Albanian rebels. Mr Aleksandar Vucic was quoted by the Tanjug news agency as accusing the US government of backing the Kosovo Liberation Army.