Iran's standoff with the West entered a critical phase yesterday with the arrival in Tehran of a high-level United Nations team demanding answers on suspected military aspects of its nuclear programme.
The three-day visit by inspectors from the International Atomic Energy Agency (IAEA) could defuse tensions that have escalated to crisis level in recent weeks.
Tehran said it was "very optimistic" about their mission, but warned it could withdraw cooperation if the experts became a "tool" for western powers.
In a possibly conciliatory signal, Iran's hardline parliament yesterday postponed debating a "double emergency" bill to immediately cut oil exports to Europe.
The pre-emptive measure was proposed in retaliation for the EU's decision last week to ban oil imports from Iran.
Brussels allowed countries such as Greece, Spain and Italy, which rely heavily on Iranian oil, until July 1 to find alternative suppliers. Iranian hardliners saw no reason to hold back while the EU took its time in a leisurely fashion to punish Iran while ensuring the bloc suffered no pain.
Cutting oil exports to Europe might cost Iran revenues but, they insisted, Tehran was ready to pay the price of being "dignified". In other words, while Iran might lose income by turning off the oil tap to Europe, it argues this is worth it to prove a matter of principle. Moreover, Tehran hopes Asian buyers will make up for any loss of sales to Europe.
Otherwise, the Iranian regime could be accused by its people of being on the ropes, unable to hit back at repeated body blows from the West.
The head of Iran's state oil company, Ahmad Qalehbani, warned yesterday that pressures on Iran's oil exports could drive prices as high as $150 a barrel.
Herman Nackaerts, the IAEA's Belgian deputy general who is leading a six-man team in Iran, said he aimed to "resolve all the outstanding issues" in the first such discussions with Tehran in more than three years. In particular, he added: "We hope that Iran will engage with us on our concerns regarding the possible military dimensions of Iran's nuclear programme".
Given deep mutual mistrust, few expect a quick breakthrough. Mr Nackaerts characterised the trip as "the start of a dialogue" that diplomats say could last months.
He is seeking Iranian explanations for evidence, described as "credible" in an IAEA report in November, pointing to past experimentation on nuclear weapons design. That report was followed by unprecedentedly harsh US and European Union sanctions targeting Iran's vital oil exports.
Iran has repeatedly threatened to block the Strait of Hormuz, the chokepoint at the mouth of the Gulf that is the global artery for oil exports, if its own sales of crude are sanctioned. The US and Britain have in turn vowed to keep the vital waterway open, by force if necessary.
The IAEA has said its information was drawn from intelligence provided by several countries and its own sources. Iran has refused to discuss the alleged weapons experiments for three years, rejecting the intelligence documents as forgeries provided by a "few arrogant countries", a reference to the US and its allies.
But ahead of the IAEA visit, Tehran said it was willing to talk about "any issues" of interest to the UN watchdog, including military-linked concerns.
Iran's envoy to the Vienna-based IAEA, Ali Asghar Soltanieh, said on Friday that the visit would help to foil enemy plots and prove the peaceful nature of his country's nuclear programme.
The IAEA team is understood to include two senior weapons experts: Jacques Baute of France and Neville Whiting of South Africa. That suggests Tehran may be prepared to address some issues related to the weapons allegations, which would be a major departure from its outright refusal to talk about them.
The inspectors are expected to call for access to sites, officials and documents that could help clarify mounting concerns that Iran may be trying to develop a nuclear arms capability.
Many western experts are deeply sceptical of Tehran's intentions. "I think they'll show Nackaerts around some facilities and show him some centrifuges and then trumpet this as them showing transparency," a European diplomat said. "They have a way of signalling some mild flexibility, which does not then appear in practice."
Tehran was equally suspicious. State-run media urged the government to be vigilant, saying some of the IAEA inspectors were "spies".
However, Peter Jenkins, a British former ambassador to the IAEA, saw a"real chance of progress". Given ever-tightening sanctions and western sabre-rattling, "this would be a very good time for Iran to try and influence public opinion, particularly in the West, by giving the IAEA inspectors something to take back to Vienna", he said.
Otherwise, Iran risks being referred once again to the UN Security Council for possible further punitive measures when the IAEA's 35-nation governing board meets in early March.
Even if, despite repeated denials, Iran conducted past research into nuclear weapons research, it would be most unlikely to admit it.
Hawkish "elements in the US that are not intent on playing this game by the book" would "raise a hue and cry… and try to get UN Security Council approval for still harsher sanctions", Mr Jenkins said.
The outcome of the IAEA visit could determine whether Iran will face further international isolation or if there are prospects for resuming wider talks with world powers on resolving the long-running and increasingly dangerous nuclear dispute.