Syria's refusal to allow UN inspectors access to a desert site where secret nuclear activity may have taken place is endangering potential evidence in the investigation, the International Atomic Energy Agency said.
It has been over two years since the IAEA was allowed to inspect the site, bombed to rubble by Israel in 2007. Syria, an ally of Iran, denies ever having an atom bomb program.
"With time, some of the necessary information may deteriorate or be lost entirely," the IAEA chief Yukiya Amano wrote in a confidential report obtained by Reuters.
US intelligence reports have said the site, known as either al-Kibar or Dair Alzour, was a nascent North Korean-designed nuclear reactor to produce bomb fuel.
Earlier this year the IAEA gave some weight to suspicions of illicit atomic activity at the site by saying that uranium traces found in a 2008 visit by inspectors pointed to nuclear-related activity.
"The features of the building and its connectivity to adequate cooling are similar to what may be found at a nuclear site," the latest report said.
The agency wants to re-examine the site so it can take samples from rubble removed immediately after the air strike.
Amano urged Syria to cooperate and criticized it for failing to provide documents related to Dair Alzour and making only statements "limited in detail" about it.
He also repeated a call for IAEA access to three other Syrian sites under military control whose appearance was altered by landscaping after inspectors asked for access.
Washington's envoy to the IAEA said last month a "number of countries" were beginning to ask whether it was time to invoke the IAEA's "special inspection" mechanism to give it the authority to look anywhere necessary in Syria at short notice.
The agency last resorted to special inspection powers in 1993 in North Korea, which still withheld access and later developed nuclear bomb capacity in secret.
The IAEA lacks legal means to get Syria to open up because the country's basic safeguards treaty covers only its one declared atomic facility, an old research reactor.
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