The UN nuclear watchdog said on Wednesday there had been "no progress" in talks with Tehran on disputes over the monitoring of Iran's atomic program.
Rafael Grossi, director general of the International Atomic Energy Agency (IAEA), told a quarterly meeting of the agency's board that talks he held in Tehran on Tuesday were "inconclusive" despite being "constructive", according to the AFP news agency.
Grossi had sought to tackle constraints put on IAEA inspections earlier this year, outstanding questions over the presence of undeclared nuclear material at sites in Iran, and the treatment of IAEA staff in the country.
"In terms of the substance... we were not able to make progress," Grossi told reporters, saying that the lack of agreement had come "in spite of my best efforts".
Behrouz Kamalvandi, spokesman for the Atomic Energy Organization of Iran, told Iranian television his team "tried until the last moment" but there is still work to be done.
Iran said last week it had invited Grossi for talks after he expressed concern over a lack of contact with Iranian authorities.
Days before the invitation, the IAEA chief said it was “astonishing” that he has had no contact with the new Iranian government over several important outstanding issues since it took office.
Grossi visited Iran in September, where he reached an agreement with the government over access to surveillance equipment at Iranian nuclear facilities.
His latest visit to Tehran came ahead of the scheduled resumption on Monday of negotiations between Iran and world powers aimed at reviving the 2015 nuclear deal.
The United States said it was "disappointed" by the outcome of Grossi's visit and said it was ready to negotiate in Vienna.
"But of course Iran's failure to cooperate is a bad sign about their seriousness in a successful conclusion to our negotiations," a State Department spokesperson said.