The UN nuclear monitoring agency cannot exclude the possibility that Iran may be hiding atomic activities, the agency's head said Monday. Mohamed ElBaradei also acknowledged deadlock in a probe of alleged attempts by Tehran to make nuclear arms. On Syria, which is also the focus of a nuclear investigation, ElBaradei suggested that Damascus seemed to be stonewalling efforts by his International Atomic Energy Agency to establish whether it tried to secretly build a plutonium producing reactor. Neither observation was new but were noteworthy in reflecting how the IAEA was being stymied in its investigations of the two countries. Iran is under UN Security Council sanctions for refusing to freeze uranium enrichment, which can be used both to make nuclear fuel or the fissile core of warheads. It additionally is suspected of at least planning to make nuclear weapons, according to intelligence supplied by the United States, Israel and other IAEA member nations. Syria came into the nuclear spotlight in 2007 after Israeli jets bombed a site that Washington says was a nearly built nuclear reactor. Samples taken from the targeted building have revealed traces of man-made uranium, and the agency is also following up on suspected Syrian purchases of materials that could be used for nuclear purposes. ElBaradei urged both nations to show increased cooperation, in comments to the opening session of the IAEA's 35-nation board. Unless Iran opens up its nuclear activities to greater agency examination, the IAEA "will not be in a position to provide credible assurances about the absence of undeclared nuclear material and activities," he said. Additionally, the agency "was unable to make any progress on ... concerns about possible military dimensions of Iran's nuclear program because of lack of cooperation by Iran," he said. On Syria - which has blocked repeated IAEA requests for new inspections - ElBaradei said Damascus has either ignored questions asked by agency experts on the nature of its nuclear activities or provided only partial information that in some cases was already known. The US also focused on Syria and Iran at the meeting Monday, in comments reflecting the new administration's willingness to negotiate with both nations after years of confrontation under the former presidency.