The outgoing head of the International Atomic Energy Agency said he had reached a "dead end" investigating Iran's nuclear programme.
Mohamed ElBaradei also said Tehran's credibility was shrinking after its belated admission that it was secretly building a nuclear facility.
Mr ElBaradei's blunt criticism yesterday of the Islamic Republic - four days before he leaves office - was notable in representing a broad convergence with Washington's opinion, which for years was critical of the IAEA chief for what it perceived as his softness on Iran.
Iran also came in for censure from another quarter at the opening session in Vienna, Austria, of the IAEA's 35-nation board, with the introduction of a resolution taking Tehran to task on a broad range of issues linked to international concerns that it may be seeking to make nuclear weapons.
Significantly, diplomats at the meeting said the resolution was endorsed not only by Western powers - the US, Britain, France and Germany - but also by Russia and China.
For strategic and economic reasons, Moscow and Beijing have sided with Tehran in the past, preventing several Western attempts to slap new United Nations sanctions on Iran for its nuclear defiance, or watering down their severity.
They did not formally endorse the last IAEA resolution critical of Iran in 2006 and their backing for the document at the Vienna meeting yesterday reflected broad international disenchantment with Tehran.
It also appeared to signal possible support for any new Western push for a fourth set of UN Security Council sanctions, should Tehran continue shunning international overtures meant to reach agreements that reduce concerns about its nuclear ambitions.
In Tehran, state TV quoted Iran's envoy to the UN agency, Ali Asghar Soltanieh, as saying: "The Western countries should not spoil the positive atmosphere. They should allow co-operation between Iran and the agency to continue its positive trend."
The IAEA resolution criticised Iran for defying a security council ban on uranium enrichment - the source of both nuclear fuel and the fissile core of warheads.
It also censured it for secretly building a uranium enrichment facility and demanded that it immediately suspend further construction, noted that Mr ElBaradei could not confirm that Tehran's nuclear programme was exclusively geared towards peaceful uses, and expressed "serious concern" that Iranian stonewalling of an IAEA probe meant "the possibility of military dimensions to Iran's nuclear programme" could not be excluded.
Western diplomats said they expected about two-thirds of the board to support the resolution in a vote, expected to take place today.
While the board cannot enforce any of its resolutions, they do get referred to the security council, giving any later move to impose new UN sanctions on Iran additional weight.
In his comments, Mr ElBaradei touched on the same criticisms expressed in the resolution.
"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 programme," he told the board session.
"We have effectively reached a dead end, unless Iran engages fully with us."
"Issues of concern" is the IAEA term for indications that Tehran has experimented with nuclear weapons programmes, including missile-delivery systems and tests of explosives that could serve as nuclear-bomb detonators.
Mr ElBaradei has emphasised the need for talks instead of threats in engaging Iran. He has criticised the US for invading Iraq on the pretext that Saddam Hussein had a nuclear weapons programme and that - and perceived softness on the Iran issue - has drawn criticism from the US and its allies that he was overstepping his mandate.
But Mr ElBaradei's comments yesterday left little doubt that he was most unhappy with Tehran.
"I am disappointed that Iran has not so far agreed" to a proposal involving removal of most of Iran's enriched stockpile, he told the meeting.
The plan approved by the six world powers negotiating with Iran over the past few months would commit Tehran to ship out 70% of its enriched uranium for processing into fuel rods for its research reactor in Tehran.
That would help allay international fears by removing most of the material that Iran could use to make a nuclear weapon.