By Fredrik Dahl
VIENNA (Reuters) - The United States warned Syria Wednesday it "can't be allowed" to stonewall a U.N. watchdog investigation into a desert site where covert atomic activity may have taken place before it was destroyed by Israel in 2007.
For more than two years Syria has refused the International Atomic Energy Agency (IAEA) follow-up access to the Dair Alzour site that U.S. intelligence reports said was a nascent North Korean-designed nuclear reactor, intended to produce bomb fuel.
The complex was bombed to rubble by Israel in 2007. Syria, an ally of Iran, denies ever having an atom bomb program.
"The United States position on this is that we are not going to let this matter simply fade away or go away," U.S. Ambassador Glyn Davies told reporters on the sidelines of a week-long meeting of the IAEA's 35-nation governing board.
He suggested the Arab state may still be pursuing secret atomic work, accusing it of "deliberate efforts to conceal the full extent and scope of what we strongly believe were, and may still be, clandestine nuclear activities."
Syria denies ever concealing work on nuclear weapons and says the Vienna-based agency should focus on Israel instead because of its undeclared nuclear arsenal.
Damascus has suggested the uranium traces came with Israeli munitions used in the attack, an assertion the U.N. agency has dismissed as unlikely.
President Bashar al-Assad said in a Wall Street Journal interview in January that Syria would not grant IAEA inspectors unrestricted access to possible nuclear sites because it would amount to a violation of sovereignty.
Davies said the Syrian case represented a challenge to the IAEA's nuclear safeguards regime. "They can't be allowed to simply stonewall and block the investigation."
IAEA Director General Yukiya Amano said Monday he saw possible movement in the Syrian probe, referring to a statement from Damascus it would work with the U.N. body to resolve all outstanding technical questions.
Syria last week agreed to allow U.N. inspectors to visit an acid purification facility where uranium concentrates, or yellowcake, were a by-product.
But letting inspectors only go to the Homs plant would not satisfy Western concerns about Syria, which are focused on Dair Alzour and sites linked to it.
The United States has suggested the IAEA may need to consider invoking its "special inspection" mechanism to give it authority to look anywhere necessary in Syria at short notice, if Syria does not agree to inspectors visiting Dair Alzour.
The agency last resorted to such inspection powers in 1993 in North Korea, which still withheld access and later developed a nuclear bomb capacity in secret.
"I think it is still the case that this is a situation that could be addressed by a special inspection," Davies said.
But nuclear expert Mark Hibbs of the Carnegie Endowment for International Peace said he did not believe Amano would do this.
"Were the IAEA to go to these sites and find nothing, Syria would be vindicated and the political credibility of the IAEA could be damaged," Hibbs said.