UN atomic watchdog chief Mohamed ElBaradei urged Iran on Monday to "unblock" a long-running nuclear standoff and expressed hope that a possible change in US policy towards Tehran may help break the deadlock.

"I again urge Iran to implement all measures required to build confidence in the exclusively peaceful nature of its nuclear programme at the earliest possible date and to unblock this stalemated situation," ElBaradei said in his opening speech to the International Atomic Energy Agency's 35-member board of governors in Vienna.

And he added: "I am hopeful that the apparent fresh approach by the international community to dialogue with Iran will give new impetus to the efforts to resolve this long-standing issue."

While the Egyptian-born diplomat did not specifically mention the United States, he was clearly referring to President Barack Obama's signal that he is ready to talk with Iran directly.

The US ambassador to the IAEA, Gregory Schulte, also addressed the first day of meeting, which wrapped up Monday and was slated to last until Thursday.

As the first meeting since Obama's inauguration in January, "this is a moment of complex challenge for the IAEA, but also a moment of unparalleled opportunity," Schulte said.

Under former president George W. Bush, relations between Washington and ElBaradei were not always easy. The United States accused ElBaradei of being too soft on Iran, while he viewed Washington's refusal to negotiate with Tehran directly as unhelpful.

"The new administration intends to strengthen diplomatic efforts to address" the challenges of Syria and Iran, Schulte said.

The two disputed dossiers were expected to be debated on Tuesday or Wednesday, diplomats said.

Even after a six-year investigation, the IAEA has been unable to say once and for all whether Iran's controversial nuclear programme is entirely peaceful, as Tehran claims.

Iran's first satellite launch in February and the announcement that its first nuclear power plant in Bushehr could go on line within months have heightened proliferation concerns in many Western countries, as has a prediction that Tehran may soon be in a position to build a nuclear bomb.

The chief of the US military, Admiral Mike Mullen, told CNN Sunday that Iran has enough fissile material for a bomb, the first time that Washington has made such an assessment.

But US Defence Secretary Robert Gates was more cautious.

"They're not close to a weapon at this point," he said.

Iranian foreign ministry spokesman Hassan Ghashghavi denied that Tehran was seeking to make a nuclear bomb.

"All this talk is baseless," he said.

Turning to the IAEA's investigation into allegations of illicit nuclear work by Syria, ElBaradei pressed Damascus to come clean on a suspect site in the remote desert, which the US alleges had been an undeclared nuclear reactor until it was bombed by Israeli planes in September 2007.

"The agency expects Syria to provide additional information and supporting documentation about the past use and nature of the building at the Dair Alzour site," ElBaradei said.

The IAEA has already said that the building bore some of the characteristics of a nuclear facility. And UN inspectors have detected "significant" traces of man-made uranium at the site, as yet unexplained by Damascus.

Syria has claimed the uranium came from the Israeli bombs, but the watchdog has more or less ruled out that interpretation.

"Our current assessment is that there is a low probability that the uranium was introduced by the use of missiles," ElBaradei said Monday.

The IAEA chief also urged Syria to provide access to additional related sites.

"Such access, together with the sampling of the destroyed and salvaged equipment and debris, is essential for the agency to complete its assessment," he said.

One of the pressing items on the agenda this week will be who is to replace ElBaradei, who is stepping down in November after 12 years in the post.

There are currently only two candidates, Japanese ambassador Yukiya Amano and South Africa's Abdul Samad Minty and both candidates were lobbying hard behind the scenes on Monday, diplomats said.