Talks with Iran on explaining the origin of processed uranium particles found at apparently old but undeclared sites are at "a very difficult juncture", UN nuclear watchdog chief Rafael Grossi said on Wednesday.
The announcement came as Grossi, who is due to report on the issue to the International Atomic Energy Agency's Board of Governors before a quarterly meeting that starts on June 6, expressed his concerns with a panel discussion at the World Economic Forum in Davos.
The Vienna negotiations are now stalled but Grossi has said 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," Grossi said.
Talks to revive Iran's 2015 nuclear deal with world powers have been on hold since March.