The International Atomic Energy Agency says a Syrian site bombed by Israel last year had features resembling those of a nuclear reactor site.
But the UN agency has not excluded a possibility the site was being used for purposes unrelated to nuclear activity.
In the first report on its findings in Syria, the IAEA said "significant" quantities of uranium were also found.
Syria maintains the site was a disused military building.
Israeli warplanes flattened the premises in the Syrian desert in September 2007 on suspicion that it was part of a covert nuclear weapons programme.
IAEA inspectors visited the bombed al-Kibar site in June, where they took samples. Damascus has blocked any follow-up trips.
The BBC reports that Syria says the uranium found there was residue from Israeli missiles used in the air-raid.
But a senior official close to the IAEA said the type of uranium found was not that usually found in munitions.
The IAEA says it wants to visit three locations in Syria to take further samples. It also intends to ask Israel to provide information about Syria's claims about the uranium particles.
IAEA head Mohamed ElBaradei said the investigation had been made more difficult by the fact that Syria has erected a new building on the site.
The US has said the target of Israel's raid was a secret nuclear reactor built with North Korean help, that was nearing completion.