Talks with Iran aimed at ending a long stand-off about explaining the origin of uranium particles found at apparently old but undeclared sites are at "a very difficult juncture", UN nuclear watchdog chief Rafael Grossi said on Wednesday.
Mr Grossi and Iran agreed in March on a three-month plan to resolve the matter, which has been a source of tension between Iran and western powers even during wider negotiations aimed at bringing Tehran and Washington back into full compliance with the 2015 Iran nuclear deal.
Those wider talks are now stalled but Mr Grossi says it is hard to imagine any agreement to revive the 2015 deal being implemented while the International Atomic Energy Agency still has not received satisfactory answers on this issue.
"I suppose I should abstain from having a final conclusion at this point since we haven't finished the process yet, but let me say that we are at a very difficult juncture at the moment," Mr Grossi told a panel discussion at the World Economic Forum in Davos, Switzerland.
Mr Grossi is due to report to the IAEA's 35-nation board of governors on how talks on the open issues have progressed by the time the board starts a quarterly meeting on June 6.
"I hope that the time ... between now and the issuance of my report will [be] put to good use to come [up] at least with a start of a credible answer to these things."
While Mr Grossi's effort to obtain answers from Iran is not part of the wider talks to revive the 2015 deal, a lack of progress could lead to renewed confrontation between Iran and the West at the board that would only complicate the indirect talks between Iran and the United States that were last held in March.