The UN's chief nuclear inspector arrived in Iran on Sunday on a mission to clear up "outstanding substantive issues" on Tehran's atomic programme, and called for dialogue with the Islamic state.
Before departing from Vienna airport, International Atomic Energy Agency (IAEA) chief inspector Herman Nackaerts told reporters that talks were long overdue.
"We are trying to resolve all the outstanding issues with Iran," he said.
"In particular we hope that Iran will engage with us on the possible military dimensions of Iran's nuclear programme. We are looking forward to the start of a dialogue, a dialogue that is overdue since very long."
Nackaerts is leading a six-person IAEA team due to meet Iranian officials from later Sunday until Tuesday. The delegation touched down in Tehran early on Sunday morning, the official news agency IRNA reported.
The team also includes IAEA number two Rafael Grossi, an Argentine, and the watchdog's senior legal official Peri Lynne Johnson, a US citizen, according to diplomats.
Nackaerts, who is Belgian, declined to comment on who he would meet during the trip, which is aimed at clearing up what the IAEA called "outstanding substantive issues" on Tehran's nuclear programme.
Expectations are low, with the delegation not expected to be given access to any sites mentioned in a damning IAEA report in November that raised suspicions Iran had done work developing nuclear weapons.
IRNA quoted Iranian Foreign Minister Ali Akbar Salehi, in Addis Ababa for an African Union meeting, as saying he was "optimistic" about the delegation's visit.
"We have always had a broad and close cooperation with the agency and we have always maintained transparency as one of our principles working with the agency," he said.
The agency added that the team would probably visit the Fordo enrichment site south of the capital Tehran.
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Earlier this month, the IAEA said Iran had begun enriching uranium to 20 percent purity deep inside a mountain bunker at Fordo, taking it significantly closer to the 90 percent mark needed for a nuclear bomb.
With Iran repeatedly denying it wants nuclear weapons and dismissing the IAEA report as baseless, the watchdog's chief Yukiya Amano on Friday urged the Islamic republic to show "substantial cooperation" during the visit.
The report, which has led to a substantial increase in pressure on Iran from the United States, the European Union and others, detailed a string of areas in which it said Iranian activities were highly suspicious.
"We have information that indicates that Iran has engaged in activities relevant to the development of a nuclear explosive device," Amano said on Friday at the World Economic Forum in Davos, Switzerland.
"We are requesting that Iran clarifies the situation. We proposed to make a mission and they agreed to accept the mission. The preparations have gone well but we need to see what actually happens when the mission arrives."
Iran's President Mahmoud Ahmadinejad has insisted that Tehran is not dodging negotiations and was ready to sit down with world powers Britain, China, France, Russia, the United States, and Germany for talks.
The six powers are waiting for Tehran to reply to an October letter sent by EU foreign policy chief Catherine Ashton that stresses that discussions should focus on the "key question" of the Iranian nuclear issue.
IRNA said Sunday that Iran's top nuclear negotiator Saeed Jalili will write a letter to Ashton which "will give Iran's views about the time and venue of the (next) negotiations".
The letter "which may be sent soon, in the coming days", it quoted the foreign minister as saying, adding the next round of talks were likely to be a "success since both sides are interested to find a solution for Iran's nuclear issue."
Previous talks held a year ago in Istanbul ended without progress.
UN chief Ban Ki-moon, also in Davos, said the onus was on Iran to prove its good intentions.
"There is no other alternative to addressing this crisis than peaceful resolution through dialogue," Ban told reporters.
What makes the international community all the more nervous is Tehran's continued defiance of UN Security Council resolutions calling on it to stop enriching uranium until the IAEA is satisfied its programme is peaceful.