The United States recently revealed its intelligence material about the suspected Syrian atomic plant, saying it was "nearing operational capability" a month before Israeli warplanes bombed it on September 6.
Mohamed ElBaradei, director of the International Atomic Energy Agency, lambasted Israel for the air strike, saying his inspectors should have been able to verify beforehand whether undeclared nuclear activity had been going on.
"In light of (this, I) view the unilateral use of force by Israel as undermining the due process of verification that is at the heart of the non-proliferation regime," he said.
ElBaradei said the U.S. allegations against Syria, which denied the U.S. charges and accused Washington of involvement in the Israeli air attack, would be investigated with due vigor.
"The Agency will treat this information with the seriousness it deserves and will investigate the veracity of the information," he said in a statement.
But ElBaradei, alluding to the United States, deplored a failure to share intelligence information "in a timely manner" about the project, which Washington said was launched in 2001.
A diplomat close to the Vienna-based agency expressed anger at the delay. "There is a lot of annoyance here about the lateness in the day that the IAEA got this information. Had this been given to the IAEA before this damn bombing, then the world might know the true story," the diplomat said.
"Even right after the bombing, before the place was totally cleaned up, it would have been easier."
ElBaradei confirmed Washington had handed over information this week saying a Syrian installation destroyed by an Israeli air strike in September was an unfinished atomic reactor.
The diplomat and analysts said the U.S. disclosure did not amount to proof of an illicit nuclear arms program since there was no sign of a reprocessing plant needed to convert spent fuel from the plant into bomb-grade plutonium.
"The absence of such facilities gives little confidence that the reactor was part of an active nuclear weapons program," David Albright and Paul Brannan of the Institute for Science and International Security said in an email commentary.
"The United States does not have any indication of how Syria would fuel this reactor... This type of reactor requires a large supply of uranium fuel. (All of this) raises questions about when this reactor could have operated."
Analysts say the Bush administration chose not to release the intelligence earlier given a risk that it might prompt Syria to retaliate against Israel.
It published the information this week, they said, in the hope that revealing what it believes about suspected Syria-North Korean nuclear cooperation would encourage Pyongyang to come clean about suspected proliferation activities and in turn encourage Congress to support dropping sanctions on North Korea.
Syria likens the U.S. allegations to those made against Iraq about weapons of mass destruction that were never found. It accused the United States of colluding in Israel's air strike.
"The U.S. administration was apparently party to the execution" of the air raid, a Syrian government statement said, without giving details. A U.S. official said Washington did not give Israel any "green light" to strike the area.
Israel is widely believed to have the Middle East's only nuclear arsenal which experts estimate at up to 200 warheads. The Jewish state has never declared its nuclear firepower as part of a "strategic ambiguity" policy to deter adversaries.
Syria has belonged to the 144-nation IAEA since 1963 and has one, declared small research reactor subject to U.N. inspection.
The diplomat close to the IAEA said it would make contact with Syrian officials soon to start pushing for explanations.
Damascus pledged to cooperate. "Syria has nothing to hide.... We are not afraid of this cooperation," Syria's U.N. envoy Bashar Ja'afari told reporters in New York.
Vienna diplomats said Syria refused earlier requests for IAEA inspectors to visit the alleged reactor site after the air raid. Syria subsequently razed and buried the installation and removed "incriminating equipment", Washington said.
"It is essential that Syria shed full light on its nuclear activities, past and present, in accordance with its international obligations," French Foreign Ministry spokeswoman Pascale Andreani told reporters in Paris.
"The clandestine construction of a nuclear reactor would be a major breach of Syria's non-proliferation obligations."
Under a deal North Korea struck with five regional powers, it had until the end of 2007 to disclose a complete list of its fissile material and nuclear weaponry as well as answer U.S. suspicions of enriching uranium and proliferating technology.
The White House said it was convinced North Korea had helped Syria to construct a clandestine nuclear reactor.
North Korea tested a nuclear device in October 2006.