VIENNA: Iran on Thursday dismissed the U.N. nuclear watchdog's work as "unprofessional" and "unfair" shortly before the two sides are due to hold talks aimed at resolving a standoff over the origin of uranium particles found at old but undeclared sites in Iran.

The issue is a thorn in the side of both Tehran and the International Atomic Energy Agency (IAEA) since the particles suggest Iran once had undeclared nuclear material at three different locations, but the IAEA has yet to obtain satisfactory answers from Iran on how the material got there or where it went.

"The statement of the Agency in its report is completely unprofessional, illusory and unfair," Iran's ambassador to the IAEA, Kazem Gharibabadi, said in a statement to a meeting of the IAEA's 35-nation Board of Governors.

Gharibabadi was referring to a passage in an IAEA report last week that said the lack of progress was seriously affecting the IAEA's ability to determine that Iran's programme is entirely peaceful, as Tehran says it is.

Failure to resolve the issue complicates efforts to restart talks aimed at bringing the United States and Iran fully back into the fold of the 2015 nuclear deal, since Washington and its allies continue to pressure Iran to give the IAEA answers.

Having obtained concessions last weekend from Iran on another issue, keeping some monitoring equipment running, IAEA chief Rafael Grossi is due to meet Iranian nuclear chief Mohammad Eslami in Vienna next week for talks on the particles.

To Iran and allies like Russia, the fact the three sites mainly seem to date back to the early 2000s and there is no indication any of the material present was enriched to a high degree means the world and the IAEA should move on.