The semiofficial ISNA news agency reported that negotiations started at the headquarters of Iran's Atomic Energy Organization. It gave no further details.
The U.N. meetings are considered an important test of Iran's willingness to address Western concerns before the possible resumption of a wider dialogue with the U.S. and other world powers. Negotiations with the six nations the United States, Russia, China, Britain, France and Germany fell apart more than six months ago. Iran has proposed getting them back on track, perhaps as soon as later this month.
The U.S. and others hope the talks will result in an agreement that will require Iran to stop enriching uranium to a higher level that could be turned relatively quickly into the warhead-grade material.
Iran denies such aspirations, insisting it is enriching only to make reactor fuel and isotopes for medical purposes.
Iran is under tough Western oil and banking sanctions over its refusal to halt uranium enrichment.
ISNA said European Union foreign policy chief Catherine Ashton has agreed restart the next round of world power talks with Iran on Jan. 28-29, but no decision has yet been made on the venue. The last round, in Moscow last June, ended in stalemate.
The official IRNA news agency, however, said the talks may not resume until early February.
Before departing for Iran on Tuesday, U.N. team leader Herman Nackaerts said the International Atomic Energy Agency hoped to "finalize the structured approach" that would outline what the agency can and cannot do in its investigation.
IAEA spokeswoman Gill Tudor said the talks would continue Thursday.
The U.N. nuclear watchdog wants to revisit Parchin, a military site southeast of Tehran, to look into allegations that Iran may have tested components needed to develop a nuclear weapon. Tehran has steadfastly denied any such activity.
Iran says the IAEA's suspicions are based on forged intelligence provided by the CIA, Israel's Mossad, Britain's MI-6 and other intelligence agencies, and that Tehran has not been allowed to see the materials to respond to them.
The IAEA also is trying to follow up on other suspicions, including whether Iran did computer modeling of a nuclear warhead core. The agency says it has intelligence information indicating Iran carried out preparatory work for a nuclear weapons test, and development of a nuclear payload for Iran's Shahab 3 intermediate range missile a weapon that can reach Israel and Europe.
Iranians say they have bitter memories of allowing IAEA inspections and providing replies to a long list of queries over its nuclear program in the past decade. Now, Tehran says such queries should not be revived.
Iranian Foreign Ministry spokesman Ramin Mehmanparast said Tuesday that the Islamic Republic provided detailed explanations to IAEA questions on six outstanding issues in the past, but instead of giving Iran a clean bill of health, the agency leveled new allegations on the basis of "alleged studies" provided by Iran's enemies.
Iran uses that term to refer to allegations about Parchin and other claims that it says the IAEA levels only to keep the issue alive.
Tehran has twice allowed IAEA inspectors into Parchin, but now it says any new agency investigation must be governed by an agreement that lays out the scope of such a probe.
"Obligations of the other party must be clearly specified. If a claim is to be raised on a spot in Iran every day and (the U.N. agency) seeks to visit our military facilities under such a pretext ... this issue will be unending," Mehmanparast said Tuesday.
President Mahmoud Ahmadinejad acknowledged Wednesday that sanctions have slowed down Iran's growth and disrupted its foreign trade and said the country must move away from a dependence on oil revenues to overcome sanctions.
Addressing parliament, Ahmadinejad said "structural changes" are needed in Iran's economy to counter the sanctions.
Associated Press writer George Jahn contributed from Vienna.