Director General of the International Atomic Energy Agency, IAEA, Mohamed ElBaradei, expressed disappointment in Iran over its unwillingness to co-operate with his agency over concerns relating to the country's nuclear program. (Hans Punz/Associated Press)

The outgoing head of the International Atomic Energy Agency said his probe into allegations that Iran has tried to make nuclear weapons is at "a dead end" because Tehran is not co-operating.

IAEA chief Mohamed ElBaradei, speaking at the United Nations agency's 35-country board meeting on Thursday in Vienna, was also critical of Iran for trying to change a negotiated deal involving its nuclear program endorsed by six world powers.

"There has been no movement on remaining issues of concern which need to be clarified for the agency to verify the exclusively peaceful nature of Iran's nuclear program," ElBaradei told the opening session of the IAEA's board of governors.

"We have effectively reached a dead end, unless Iran engages fully with us."

ElBaradei's blunt assessment comes four days after he ends his tenure as the agency's head, and reflect the frustration the agency has had in the back-and-forth with Iranian officials over its nuclear program.

Questions about secret facilities

Revelations eight years ago that Iran was engaged in uranium enrichment and Iran's unwillingness to allow full investigations of the facilities prompted a series of UN sanctions designed to push the country to abandon its program.

But Iran has remained defiant, and says it is enriching uranium only to power a future network of nuclear reactors. But because enrichment can also produce fissile warhead material, its program has raised concerns with the United States and its allies.

Iran notified the IAEA of a second previously unknown enrichment facility in September, prompting a new round of talks in which Iran agreed to allow inspectors to visit the facility in October.

ElBaradei said Iran's decision not to inform the agency about the second facility near the city of Qom was "outside the law" and had reduced confidence that there were no other secret facilities.

Talks with Iranian diplomats also produced a UN-proposed draft agreement in which Iran would ship its available supply of enriched uranium to Russia for further enrichment, and have it returned in a form that could be used for civilian purposes.

No agreement on UN-proposed plan

The agreement had the backing of the U.S., Germany, France and Britain, as well as Russia and China.

But Tehran balked and instead proposed buying the nuclear fuel it needs for its reactors by other means.

That solution is unpalatable to the United States and its allies, who signed off on the draft plan because it would take uranium capable of being used for weapons out of Iran and replace it with fuel rods that could not be further enriched into weapons-grade material.

ElBaradei said he was most unhappy that the negotiated agreement appears to have fallen apart.

"I am disappointed that Iran has not so far agreed to the original proposal" he told the meeting. Tehran's approval "would greatly help to alleviate the concerns relating to Iran's nuclear program," he added.

With files from The Associated Press