VIENNA (AFP) - The United States accused Syria on Thursday of obstructing a UN investigation into its nuclear activities, saying the list of questions was growing. "Over one year has passed since the IAEA began investigating Syria's clandestine nuclear activities related to the destroyed reactor at Dair Alzour," US deputy chief of mission, Geoffrey Pyatt, told a closed session of the International Atomic Energy Agency's 35-member board. "Regrettably, Syria has not used this time to resolve the lingering questions about the reactor and the associated facilities... Instead, the agency's list of questions is growing," Pyatt said. Damascus had "chosen to hinder the agency's efforts, he said. "We call on Syria to cooperate fully with the agency without delay to address all unresolved questions," US diplomat Pyatt said. "We must understand why (the uranium particles) -- material that was not declared to the IAEA -- was detected at two facilities in Syria, one of which was being constructed clandestinely."