"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.

The International Atomic Energy Agency is investigating US allegations that Syria had been building a secret nuclear reactor at a remote desert site -- known alternatively as Dair Alzour or Al-Kibar -- until it was bombed by Israeli planes in September 2007.

Damascus denies the allegations and claims the site was a disused military facility.

But it has only allowed IAEA inspectors to visit the site once, and has turned down requests for follow-up inspections and access to a number of other sites.

The IAEA has already said that the building had characteristics of a nuclear facility and UN inspectors had detected "significant" traces of man-made uranium at that site, as yet unexplained by Damascus.

In its latest report on Syria, the IAEA said inspectors had now found uranium particles at a second site -- a research reactor near Damascus -- that would not normally be expected there.

It was too early to say whether the uranium particles at the two sites were connected, according to the IAEA.

But the agency has more or less ruled out Syria's claim that the uranium at Dair Alzour came from Israeli bombs.

"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 previously declared to the IAEA -- was detected at two facilities in Syria, one of which was being constructed clandestinely."