Senior United Nations nuclear inspectors headed to Tehran on Saturday to press Iranian officials to address suspicions that the Islamic state is seeking atomic weapons.
The UN International Atomic Energy Agency hopes Iran, which has indicated readiness to discuss the issue for the first time since 2008, will end years of stonewalling on intelligence pointing to an intention to develop nuclear arms technology.
"We are trying ... to resolve all the outstanding issues with Iran, in particular we hope that Iran will engage with us on our concerns regarding the possible military dimensions of Iran's nuclear programme," IAEA Deputy Director General Herman Nackaerts told reporters as he prepared to depart from Vienna airport.
But Western diplomats, who have often accused Iran of using such offers of dialogue as a stalling tactic while it presses ahead with its nuclear programme, say they doubt Tehran will show the kind of concrete cooperation the IAEA wants.
They say Iran may offer limited concessions and transparency in an attempt to ease intensifying international pressure on the country, a major oil producer, but that this is unlikely to amount to the full cooperation that is required.
The outcome could determine whether Iran will face further international isolation, or whether there are prospects for resuming wider talks between Tehran and the major powers on the nuclear dispute that has sparked fears of war.
The United States and its allies suspect the programme has military aims but Tehran says is for peaceful electricity generation.
"The chances of the IAEA's success may depend on how badly Iran wants to avoid harder sanctions," said nuclear expert Mark Hibbs of the Carnegie Endowment for International Peace.
Remarks by Supreme Leader Ayatollah Ali Khamenei's top adviser on international affairs on Saturday suggested Iran was not in the mood for concessions.
"Iran's stance towards its nuclear issue has not changed in term of fundamentals and principles," Ali Akbar Velayati said, according to the ISNA news agency.
"One important principle is that Iran would not relinquish or withdraw from its peaceful nuclear activities."
The six-member IAEA team of senior officials and experts, headed by Nackaerts, was due to arrive in Tehran early on Sunday.
The three day visit comes at a time of soaring tension between Iran and the West. The IAEA issued a report in November with details of suspected research and development activities in Iran relevant to nuclear weapons.
The West has seized on the report to ratchet up sanctions aimed at Iran's lifeblood oil exports. Iran hit back on Friday warning it may halt oil exports to Europe next week .
"APPEARING TO COOPERATE"
The IAEA team is expected to seek explanations to the issues raised in the report, including information that Iran appears to have worked on a nuclear weapon design, and demand access to sites, officials and documents relevant to the agency's probe.
The IAEA says Iran, which has rejected the allegations as forged and baseless, has not engaged with the agency in a substantive way on these issues since August 2008 and that it keeps receiving intelligence data adding to its concerns.
"There were a huge number of questions raised by the November report. They will be seeking to answer those questions, and it's incumbent on Iran to be supportive," U.S. State Department spokeswoman Victoria Nuland said this week.
IAEA Director General Yukiya Amano has called on Iran to show a "constructive spirit" in the meeting and Iran has said it is willing to discuss "any issues" of interest to the U.N. agency, including the military-linked concerns.
Iran's Press TV state television said on its website the IAEA visit was aimed at bolstering cooperation between the two sides "by resolving ambiguities", language Tehran has also used in the past.
The English-language station cited Iran's envoy to the IAEA, Ali Asghar Soltanieh, as saying the main objective was to "thwart plots by enemies who are levelling unfounded allegations" against Iran and to prove its nuclear transparency.
Hibbs said Amano would want to see a "significant step" from Iran, for example by agreeing to more intrusive IAEA inspections or by explaining issues related to the weapons suspicions.
"I'm not very optimistic," Hibbs said. "Iran's track record is of appearing to cooperate whenever they are threatened by penalties."