Iran and a team from the U.N. nuclear watchdog started a fresh round of talks on Monday in Tehran to resolve doubts about the Islamic Republic's nuclear work, Iranian media reported.
The International Atomic Energy Agency (IAEA) delegation arrived in Iran's capital on Sunday, less than a week after a U.S. intelligence report said Iran halted its nuclear weapons program in 2003.
Iran denies ever having had such a military program but welcomed the report that contradicted the U.S. administration's assertions that Tehran was actively working on a nuclear bomb.
The nuclear negotiations started on Monday morning and will last three days, the semi-official news agency Fars reported.
Iran and the U.N. body agreed in August on a timetable to answer outstanding questions about nuclear activities which Tehran says are aimed at generating electricity.
Previous rounds of talks dealt with centrifuges used to enrich uranium and other issues. The new talks are expected to focus on questions about particles of arms-grade enriched uranium found by IAEA inspectors at Tehran's Technical University.
The talks will be focused on the source of contamination, the report said without elaborating.
Enriched uranium can be used both for fuelling power plants and, if refined much further, for making bombs. But Iran says it wants to refine uranium only as an alternative source of electricity so it can export more of its oil and gas.
The IAEA said in a report last month Tehran was cooperating but not proactively. IAEA director Mohamed ElBaradei said Iran was making good progress in solving questions about its plans.
The U.N. Security Council has imposed two sets of limited sanctions on Tehran for its refusal to halt enrichment, the part of Iran's program that most worries the West.
Last week's U.S. report released by the 16 intelligence agencies is expected to complicate U.S. efforts to push through new U.N. sanctions against Iran over its atomic work.
U.S. President George W. Bush said after the report that Iran still remained a danger because it was mastering technology with a military use.
European Union foreign policy chief Javier Solana told reporters in Brussels that the bloc would pursue its line of offering negotiations to Iran over inducements to halt uranium enrichment while backing moves towards U.N. sanctions.
(Writing by Reza Derakhshi; Editing by Charles Dick)