The International Atomic Energy Agency gathered in Vienna on Monday to examine Iran's nuclear defiance and suspicions that Syria secretly might have tried to build a nuclear reactor.

At the start of the five-day meeting, the UN nuclear agency's 35 board member nations were briefed on the latest developments regarding the two nations by director-general Mohamed ElBaradei.

The UN nuclear chief criticised Iran for stonewalling his investigations into allegations that Tehran worked on a secret nuclear weapons programme.

Repeating comments he first made in a report last week with even more clarity, he said Iran had not "provided the agency with all the access to documents and to individuals requested by the secretariat, nor has Iran provided the substantive explanations required to support its statements".

He said such clarifications were "critical to an assessment of the nature of Iran's past and present nuclear programme".

"The agency understands that Iran may have additional information, in particular on high explosives testing and missile related activities, which could shed more light on the nature of the alleged activities," the IAEA director-general continued.

ElBaradei also said his agency "understands that Iran may have additional information" it was withholding from IAEA experts - an allegation also made in his report last week to the agency board and the UN Security Council.

He called on Tehran to provide full access to the necessary information as soon as possible.

"It is essential that the agency be able to reach a conclusion regarding the nature of Iran's programme at the earliest possible date. This depends primarily on Iran demonstrating the necessary transparency and providing full disclosure," he explained.

Since launching its probe into the allegations last year, the IAEA has asked - in vain - for substantive explanations for what seem to be draft plans to refit missiles with nuclear warheads; explosives tests that could be used for a nuclear detonation; military and civilian nuclear links and a drawing showing how to mold uranium metal into the shape of warheads.

Iran remains defiant, dismissing evidence from the US and other board members purportedly backing the allegations as fabricated.

It is also under fire for defying three sets of Security Council sanctions and continuing to enrich uranium - a process that can generate both nuclear fuel and the fissile material for the core of nuclear warheads.

Focusing on Syria, ElBaradei said Damascus had agreed to let his inspectors into the country between June 22 and 24 to probe allegations of illegal nuclear activity.

The UN agency chief said his experts would investigate a site that Israeli warplanes destroyed in September.

The US says the strike was on a plutonium-producing reactor that was being built secretly with North Korean help.

Criticism for US

In his address to the 35-nation IAEA board, ElBaradei criticised Washington for waiting so long to brief him on its suspicions and Jerusalem for its air strike.

""It is deeply regrettable that information concerning this installation was not provided to the agency in a timely manner and that force was resorted to unilaterally before the agency was given an opportunity to establish the facts," he said.

Although the June visit will focus on what it was that the Israelis levelled in a remote section of Syria's eastern desert in September 2007, it could be the start of a larger nuclear probe.

Diplomats believe experts of the IAEA were also interested in at least two - and possibly three - other locations with possible undeclared nuclear.

The diplomats, who demanded anonymity because of the sensitivity of the information, told The Associated Press that agency experts would be asking for information on the possible existence of two plutonium reprocessing facilities.

Plutonium reprocessing capabilities would be required if the US and Israel are correct in their claims that Syria was trying to build the reactor to gain the
One of the diplomats said the agency was tipped off by the US, which had apparently gathered intelligence on the locations of the alleged facilities and other details.

The diplomat emphasised the IAEA had not seen the intelligence itself.

After ElBaradei's briefing, the US ambassador to the IAEA spoke to AP Television about his concerns regarding Iran's nuclear defiance.

"This is a matter, as Dr Elbaradei has said, of serious concern. And the IAEA board of governors expects Iran to cooperate fully with the IAEA and to fully disclose its past and current nuclear activities," Gregory Schulte said.

He said the US was willing to enter face-to-face talks with Tehran to come to a solution

"Secretary Rice has said for the last two years that she is ready, personally, to sit down at the table with the Iranian foreign minister and to discuss these issues," he said. But he stressed Iran would have to meet certain conditions for the talks to be able to take place.

"All they need to do is they need to suspend their enrichment of uranium which they don't need for civil purposes but they do need if they want to build a bomb. We'd also like to see Iran be much more serious here in Vienna in terms of their cooperation with the IAEA," the ambassador said.