Olli Heinonen, the International Atomic Energy Agency's (IAEA) deputy director general, held two sessions of talks with Iranian officials Tuesday after a first round on Monday, the state broadcaster reported.
No information filtered out on the contents of the discussions amid an apparent media blackout and neither photographs nor video footage of the meetings were released.
Heinonen's two-day visit is aimed at pressing Iran over claims it has carried out so-called "weaponization studies", the Vienna-based watchdog has said.
The deputy head of Iran's atomic energy organisation Mohammad Saeedi and its ambassador to the IAEA Ali Asghar Soltanieh led the Iranian delegation at Tuesday's talks as they had on Monday, the student ISNA news agency reported.
Official media reports before the talks had said that deputy national security chief Javad Vaeedi would lead the Iranian delegation but, as on the previous day, he was not present on Tuesday.
No reason was given for Vaeedi's absence.
ISNA said the final round of talks ended Tuesday evening without either side making any declaration.
In a sign of the sensitivity of the talks, Iran's leading hardline daily Kayhan launched a withering personal onslaught against Heinonen and his intentions on Monday.
"This trip is to complete a joint Israeli-US trick to provide phoney proof on Iran's nuclear activities," said an editorial signed by chief editor Hossein Shariatmadari, who is appointed by supreme leader Ayatollah Ali Khamenei.
In a closed-door briefing to diplomats at IAEA headquarters in Vienna on February 25, Heinonen presented detailed evidence suggesting that Iran could have been studying how to use its nuclear technology to make a warhead.
Some of the information is reported to have come from IAEA member states, including data from a laptop computer smuggled out of Tehran in an operation by Western intelligence in 2004.
Western diplomats present at the meeting subsequently said the new evidence of alleged "weaponization studies" was troubling.
Iran, which insists its nuclear programme is entirely peaceful and aimed solely at generating energy, at the time furiously denounced the claims as fake.
Iran's refusal to suspend sensitive uranium enrichment operations -- which the West fears could be used to make a nuclear weapon -- has already led to three sets of UN Security Council sanctions against Tehran.
Despite more than four years of intensive investigation, the IAEA has never been able to confirm that the nuclear drive is peaceful and bring its probe to a conclusion.
Iran has stuck to a conspicuously different characterisation of Heinonen's visit than that of the IAEA, saying it is a routine trip as part of the cooperation between Tehran and the nuclear watchdog.