(Adds Iranian, U.S. remarks at IAEA meeting)
* Iran: suspicions of bomb tests, cover-up are "ridiculous"
* Western powers, Russia, China show unity before Iran talks
* Khamenei hails Obama remarks on "diplomatic opportunity"
* But says Obama's harsh sanctions against Iran won't work
* Israel seeks advanced U.S. bunker buster bombs to hit Iran
By Fredrik Dahl
VIENNA, March 8 Six world powers demanded on Thursday that Iran fulfil a promise to let international inspectors visit a military installation where the U.N. nuclear watchdog believes that explosives tests geared to developing atomic bombs may have taken place.
The joint call demonstrated unusual unity among the powers on Iran ahead of a planned revival of high-level talks as well as widening disquiet about the nature of Tehran's nuclear quest, with Israel threatening last-ditch military action.
Iranian Supreme Leader Ayatollah Ali Khamenei welcomed comments by U.S. President Obama about a diplomatic "window of opportunity" offered by renewed talks, but said Washington's simultaneous moves to "bring the Iranian people to their knees" with harsh sanctions were driven by delusion.
Heaping pressure on Iran to come clean on its nuclear activity, the United States, Russia, China, Britain, France and Germany used a U.N. nuclear watchdog governors' meeting to urge Tehran to grant prompt access to its Parchin military facility.
They voiced concerned that no deal was reached between Iran and International Atomic Energy Agency inspectors at talks in January and February, "including on the access to relevant sites in Iran, requested by the agency ... In that context we urge Iran to fulfil its undertaking to grant access to Parchin."
Iran has said inspectors can go to Parchin, but only after a broader deal is reached on how to address all outstanding issues between Tehran and the U.N. agency -- an approach Western diplomats dismissed as a procedural stalling tactic.
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.
Robert Wood, acting U.S. envoy to the IAEA, said the powers had signalled to Iran that it was "on notice to comply with its obligations" and he suggested agency governors may take further action if Tehran did nothing before they reconvened in June.
GETTING RID OF TELLTALE EVIDENCE?
Western diplomats briefed by a senior IAEA official 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.
They referred to a recent sequence of satellite pictures of the Parchin complex showing apparent changes to its structure.
The six powers made no mention of "sanitizing" the Parchin premises in their statement at a closed-door session of the 35-nation International Atomic Energy Agency governing board.
But their language regarding Parchin and other aspects of Iran's shadowy nuclear programme sent a message to Tehran of a cohesive stance within the group on how to diplomatically tackle the long-running stand-off, which has stirred fears of war that could inflame the Middle East and send oil prices skyrocketing.
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, now facing sanctions targeting its oil exports for defying international demands to curb its nuclear activities, denies suspicions of a camouflaged bid to develop atom bombs, insisting it wants nuclear power for electricity generation.
But Israel, feeling in mortal danger from Iran's nuclear advances, openly doubts sanctions and diplomacy will rein in its arch-enemy's nuclear activity and is speaking more stridently of resorting to pre-emptive bombings of Iranian nuclear sites.
Continuing the war of words, Soltanieh warned that any Israeli attack on Iranian atomic sites would lead to the collapse of the nuclear Non-Proliferation Treaty (NPT).
Iran is a party to the 1970 pact aimed at preventing the spread of nuclear weapons. But some analysts believe Iran may quit the NPT if attacked and race to build atomic bombs. Israel is not an NPT member and is believed to harbour the Middle East's only nuclear arsenal.
"Inevitably attacks on safeguarded nuclear installations of a party to the NPT by a non-party shall lead to the collapse of the NPT," Soltanieh said in a speech to the IAEA governors.
REVIVING BIG POWER TALKS WITH IRAN
Temporarily quieting the sabre-rattling, the European Union's foreign policy chief said on Tuesday the six powers had accepted Iran's offer to revive talks after a year's standstill.
To that end, the six powers said in their statement to the IAEA board read out by China's envoy: "We call on Iran to enter, without preconditions, into a sustained process of serious dialogue which will produce concrete results".
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.
Iran has long declared its nuclear work non-negotiable and rejected, on sovereignty grounds, closer international inspections and oversight as guarantees that its enrichment activity remains peaceful, as called for by Western powers.
Ahani said all parties must be realistic in their approach to negotiations and the powers should not be worried by Iran's nuclear activities. "We have to try through dialogue to resolve them (issues) and reach a compromise and in my opinion it's better not to prejudge these negotiations in advance."
KHAMENEI PRAISES, RAPS OBAMA
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 U.S. 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 U.S. 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".
"This part of his comments shows that the illusion continues," Khamenei said, according to IRNA.
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.
Khamenei has stressed that no obstacle can hinder Iran's nuclear drive, which he called a "pillar of national dignity".
Preparing for war contingencies, Israel has asked Washington for advanced "bunker-buster" bombs and refuelling planes that could improve its ability to attack Iran's subterranean nuclear sites, an Israeli official said.
"Such a request was made" around the time of Israeli Prime Minister Benjamin Netanyahu's visit to Washington for talks with Obama earlier this week, said the official.
Israel's conventional military firepower may not be enough to deliver lasting damage to Iran's distant, widely dispersed and well-fortified facilities, many experts say.
Reflecting its concern that Islamist militants could attack Israel in retaliation for any military action against Iran by tunnelling in from neighbouring Lebanon or Palestinian territories, Israel is training its troops to hunt below ground with robot probes and sniffer dogs.
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".
IAEA chief Yukiya Amano this week said there were indications of unspecified "activities" at Parchin and that this made the agency want to visit the site sooner rather than later.
Suspicions about activities at the Parchin complex date back to at least 2004, when a prominent nuclear expert said satellite images showed it might be a site for nuclear weapons research.
U.N. inspectors did in fact visit Parchin in 2005. But they did not see the place where the IAEA now believes the explosives chamber was built.
Iran, one of the world's largest oil producers, has dismissed intelligence suggesting it has a nuclear weapons, rather than nuclear energy, agenda as forged and baseless. (Additional reporting by John Irish in Paris, Marcus George in Dubai, Mayaan Lubell in Jerusalem; Writing by Mark Heinrich; Editing by Peter Millership)