VIENNA (Reuters) - Offering immunity or an easing of the sanctions pressure may be the only way - if there is one at all - to coax Iran to end years of stonewalling a U.N. watchdog investigation into suspected nuclear weapons research in the Islamic state.
Any such initiative would likely need to come from world powers as part of a broader diplomatic thrust to defuse the dispute over Iran's nuclear programme, leaving the investigation by the U.N. atomic agency dependent on how those talks develop.
The International Atomic Energy Agency (IAEA) has failed in a series of high-profile rounds of discussions in the last six months to persuade Tehran to give it access to sites, officials and documents it says it needs for the long-stalled inquiry.
The roller-coaster negotiations have underlined the IAEA's limited power to make Iran cooperate with it, suggesting Tehran will do so only if it gets something in return elsewhere and fuelling Western suspicions that it is playing for time.
"It looks to me now that the IAEA-Iran track isn't going to go anywhere unless there is progress made in the talks between Iran and the powers," senior researcher Shannon Kile of the Stockholm International Peace Research Institute (SIPRI) said.
Iran seems to be using its discussions with the IAEA - at times raising hopes for a deal, then dashing them - to gain leverage in its separate meetings with the powers that have made little headway since they resumed in April after a 15-month gap.
The six powers - the United States, France, Russia, Germany, Britain and China - also want Iran's full cooperation with the U.N. watchdog. But their more immediate demand is that Iran stop atomic activity that takes it closer to potential bomb material.
Tehran may also require assurances that, if it eventually does agree to give U.N. inspectors greater freedom to carry out their work, any incriminating evidence they unearth will not be used against it. Iran denies Western allegations it is seeking to develop the capability to make atom bombs.
To help break the deadlock, Iran should be given "a grace period with no adverse consequences in case their full transparency with IAEA inspectors reveal past wrongdoing," said former chief U.N. nuclear inspector Pierre Goldschmidt.
Goldschmidt, now at the Carnegie Endowment for International Peace, said this should be offered and guaranteed by the powers.
"Personally I see no problem with immunity for the past," said a senior Western diplomat, who follows the nuclear issue closely but is not involved in negotiations with Tehran.
"But it has to be verifiable. The models are South Africa and Libya. I fear Iran will not accept such true transparency," the envoy said, referring to decisions years ago by those two countries to abandon their nuclear weapons ambitions.
Ali Vaez of the International Crisis Group think-tank said the relationship between Iran and the IAEA had become "hostage to the nuclear brinkmanship" of Tehran and the world powers.
The six states demand that Iran scale back its uranium enrichment programme and shut down an underground nuclear facility where it is carrying out higher-grade atomic work.
Iran seeks recognition of what it says is its legal right to refine uranium and a lifting of increasingly harsh economic sanctions now targeting its economically vital oil exports.
A bullet-point presentation of Tehran's negotiating position published by Iranian media indicated that it expects an easing of sanctions for "transparently" working with the U.N. agency.
"We are in a chicken and egg conundrum, where Iran's nuclear crisis cannot be resolved without the IAEA giving Iran a clean slate, but that will not happen until the crisis is resolved," Vaez said.
SIPRI's Kile said he believed Iran needed "something positive and tangible in return" for cooperating with the IAEA, perhaps in the area of sanctions.
The United States and its allies have ruled out offering any sanctions relief before Iran takes concrete action to ease their concerns. They have demanded that Iran halt higher-grade enrichment and close down the underground Fordow site, but without promising any significant easing of sanctions in return.
"There is another school of thought which is: Iran is simply stalling for time ... and this is basically a way of keeping the discussions going, forestalling military action and allowing their nuclear programme to advance," Kile said.
As Iran stonewalls the IAEA inquiry, Western diplomats say, satellite images show what appears to be a clean-up of a military site, Parchin, where U.N. inspectors believe Iran has carried out experiments relevant for developing nuclear weapons.
"Iran's ongoing activities at the Parchin site continue to raise concerns about efforts to destroy evidence of possible nuclear weapons-related work," a U.S. think-tank, the Institute for Science and International Security, said.
Iran has dismissed the allegations aired about Parchin, a vast military complex southeast of Tehran, as "childish" and "ridiculous", just as it rejects Western suspicions that it is seeking the capability to build nuclear bombs.
"I totally refute such accusations ... nobody can clean any nuclear contamination," Iranian Foreign Minister Ali Akbar Salehi told Reuters when asked about the clean-up allegations.
But Iran's refusal to curb nuclear work which can have both civilian and military purposes and its lack of openness with U.N. inspectors have drawn four rounds of U.N. Security Council sanctions resolutions since 2006 and separate Western measures.
The West stepped up the pressure after an IAEA report last year that revealed a trove of intelligence pointing to research activities in Iran of use in developing the technologies needed to assemble nuclear weapons, should it decide to do so.
The U.N. agency wants Iran to address questions raised by the report, such as the past alleged experiments at Parchin, and began a determined effort this year to secure Tehran's cooperation - including three visits to Tehran since January.
But when the IAEA last month hoped to finalise an accord on how to conduct the probe, Iran instead proposed amendments that would have restricted the investigation, diplomats said.
"It is back to square one," one Western envoy said.
Iran has taken the IAEA "for a ride," an ambassador said, referring to a high-profile and ultimately failed trip by IAEA Director General Yukiya Amano to Tehran in May, after which he voiced optimism about signing a deal with the country soon.
Iran's insistence that the IAEA not reopen lines of inquiry once they have been concluded was an important sticking point, diplomats said. Iran also wants access to intelligence documents forming the basis for the agency's investigation.
IAEA officials "went through such a disappointing and frustrating process last time that they would be loath to repeat that", another diplomat said about the prospects for more talks.
But Iran insists there will be more meetings with the IAEA.
Salehi said the drive to find an agreement was on track: "It may have stalled a little bit but it will speed up."
Additional reporting by Raissa Kasolowsky and Marcus George in Abu Dhabi; Editing by Mark Heinrich