Six world powers demanded Iran keep its promise to let international inspectors visit a military installation where the UN nuclear watchdog believes explosives tests geared to developing atomic bombs may have taken place.
The joint call was an unusual show of unity among the powers on Iran before a planned revival of high-level talks as well as widening disquiet about the nature of Tehran's nuclear ambitions, with Israel threatening last-ditch military action.
Iran's Supreme Leader Ayatollah Ali Khamenei welcomed comments by US President Barack Obama about a diplomatic "window of opportunity" offered by the talks, but said Washington's simultaneous moves to "bring the Iranian people to their knees" with sanctions were driven by delusion.
Heaping pressure on Iran to come clean, the United States, Russia, China, Britain, France and Germany used a UN nuclear watchdog governors' meeting on Thursday to urge Tehran to grant prompt access to its Parchin military facility.
They voiced concern that no deal was reached between Iran and International Atomic Energy Agency (IAEA) inspectors at talks in January and February, "including on the access to relevant sites in Iran, requested by the agency ... We urge Iran to fulfil its undertaking to grant access to Parchin."
The message was reinforced by a remarkably blunt statement from IAEA director Yukiya Amano accusing Tehran of seeking to "tie our hands" and restrict inspectors during their last two rounds of meetings.
His deputy Herman Nackaerts told Thursday's closed session of the IAEA board of governors session, according to one participant: "Due to major differences between Iran and the agency, agreement could not be reached."
Nackaerts, the IAEA's chief safeguards inspector, said it had information from satellite pictures showing "the precise location where we believe an explosive chamber is situated".
Iran media reports this week suggested a visit to Parchin might still be granted but the IAEA said on Thursday Tehran had not contacted it formally about any trip.
Iran's IAEA ambassador, Ali Asghar Soltanieh, told reporters outside the board meeting that the suspicions aired about Parchin were "childish" and "ridiculous". He did not elaborate.
GETTING RID OF TELLTALE EVIDENCE?
Western diplomats briefed by a senior IAEA official last week said Iran might be delaying an inspectors' trip to Parchin so that it could first clear away evidence of research with high explosives tests relevant to designing a nuclear bomb.
The six powers made no mention of "sanitising" the Parchin premises in their statement at this week's session of the 35-nation board of the IAEA, the Vienna-based UN agency.
But their language regarding Parchin and other aspects of Iran's shadowy nuclear programme sent a message to Tehran of a cohesive big-power approach to tackling the stand-off, which has stirred fears of war that could inflame the Middle East and send oil prices skyrocketing at a time of global economic downturn.
The six powers voiced "regret" about Iran's escalating campaign to enrich uranium, which can yield material for electricity or nuclear bombs and is now centred in a mountain bunker chosen as protection from air strikes.
Iran, facing sanctions targeting its oil exports for defying international demands to curb its nuclear activities, denies suspicions of a camouflaged attempt to develop atom bombs, insisting it wants nuclear power for electricity generation.
But Israel, seeing Iran's nuclear advances as a mortal threat, doubts sanctions and diplomacy will rein in its arch-enemy and is speaking more stridently of resorting to pre-emptive bombings of Iranian nuclear sites.
Prime Minister Benjamin Netanyahu said Israel would give sanctions on Iran a chance to work and would not attack its nuclear installations in the coming days or weeks.
"I am not standing with a stopwatch in hand. It is not a matter days or weeks, but also not a matter of years. Everybody understands this," he told Israeli television Channel 10.
Israel, believed to harbour the Middle East's only nuclear arsenal, has asked the United States for advanced "bunker-buster" bombs and refuelling planes that could improve its ability to attack Iran's underground nuclear sites, an Israeli official said on Thursday.
A US official confirmed Netanyahu and Defense Secretary Leon Panetta discussed military capabilities but said no deals were struck during those talks.
A US Air Force general said a 30,000-pound (13,600-kg) bunker buster bomb designed to smash through some 200 feet (65 metres) of concrete before exploding is a "great weapon" that could be used by US forces in a clash with Iran.
REVIVING BIG POWER TALKS WITH IRAN
Temporarily quieting the sabre-rattling, the European Union's foreign policy chief announced on Tuesday the powers had accepted Iran's offer to revive talks after a year's standstill.
The Islamic Republic's chief nuclear negotiator Saeed Jalili last month promised to float "new initiatives" at the talks, whose venue and date are not yet decided.
But Iran's ambassador to France, Ali Ahani, said on Thursday its "inalienable" right to enrich uranium would not be on the table - a stance redolent of past talks that ran aground over an inability to agree even on an agenda.
Obama on Monday warned against "bluster" and "loose talk of war" over Iran, which he felt had driven up oil prices, and said he was convinced "that an opportunity remains for diplomacy - backed by pressure - to succeed".
Khamenei hailed Obama's reference to opportunity. "We heard two days ago that the US president said that (they) are not thinking about war with Iran. These words are good words and an exit from delusion," Khamenei was quoted by IRNA as saying.
But Khamenei's praise for a US leader, rare for Iran's paramount conservative clerical leader, was tempered by criticism of what he called an Obama remark about "bringing the Iranian people to their knees through sanctions".
The United States has succeeded in severely limiting Iran's access to global financial services and in extending its own ban on Iranian oil to the European Union and beyond.
An IAEA report last year revealed a trove of intelligence pointing to research activities in Iran of use in developing the means and technologies needed to assemble nuclear weapons, should it decide to do so.
One salient finding was information that Iran had built a large containment chamber at Parchin in which to conduct high-explosives tests that the IAEA said are "strong indicators of possible weapon development".