A new U.S. readiness to engage Iran could help resolve suspicions about its nuclear work but Tehran must do more to "unblock this stalemated situation", the U.N. nuclear watchdog chief said on Monday.

Mohamed ElBaradei made the remarks in an address kicking off a week-long meeting of the International Atomic Energy Agency's 35-nation board of governors in Vienna, the first since U.S. President Barack Obama took office on Jan. 20.

ElBaradei also said Syria was obstructing IAEA efforts to clarify U.S. intelligence indications that it almost built a covert nuclear reactor geared to yielding plutonium for atom bombs before it was destroyed in a 2007 Israeli air strike.

Obama has signalled a readiness to talk to Iran, Syria and other U.S. foes without preconditions after years of unfruitful isolation policy by George W. Bush, but concrete steps await the outcome of a foreign policy review in about a month.

Still, Washington's return under Obama to support for multilateral cooperation to address frozen conflicts has been greeted with relief in the IAEA and by many of its governors, and ElBaradei's remarks touched on that.

"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 in a way that provides the required assurances about the peaceful nature of Iran's nuclear programme, while assuring Iran of its right to use nuclear energy for peaceful purposes," he said.

ElBaradei did not mention the Obama administration by name, but his inference was clear.

STAND-OFF

But he reiterated that Iran was stonewalling IAEA attempts to defuse mistrust in its nuclear ambitions by:

* refusing to suspend uranium enrichment, a process that can yield fuel for power plants or, if reconfigured, for atom bombs.

* refusing to allow U.N. inspector checks beyond declared nuclear installations.

* refusing to let inspectors go to a heavy water reactor under construction to verify Iranian design data to ensure it will be put only to peaceful uses.

* refusing to provide documentation, access to officials for interviews and relevant sites to check intelligence allegations of "possible military dimensions" to its nuclear fuel programme.

"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 told the gathering.

Iran says it is enriching uranium for a planned network of nuclear power plants, not weapons as Western powers suspect.

It says the mainly U.S. intelligence is forged and the access the IAEA is seeking involves classified information and military sites of a conventional, not nuclear, nature and therefore beyond the agency's scope.

Iran stopped allowing wider-ranging inspector movements in retaliation for U.N. sanctions it calls unjust and illegal.

On Syria, IAEA inspectors say enough traces of uranium were found in soil samples taken in one trip to the bombed site granted by Syria last June to constitute a "significant" find, and satellite pictures revealed a building resembling a reactor.

ElBaradei said Syria repeated in a Feb. 15 letter to the IAEA that the wrecked facility, and another built on top of it, were military installations not involved in nuclear activities.

"The letter did not address many of the questions (we) raised. Syria's responses to some of (our) questions were only partial and included information already provided," he said.

He again pressed Syria to back up its denials of wrongdoing with documentation and more on-the-ground IAEA access.

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

Diplomats close to the IAEA say satellite imagery shows Syria removed such materials and landscaped sites in question to alter their appearance after inspectors asked to see them.