Iran on Thursday accused the UN nuclear agency of "a counter-productive approach" after its chief said Tehran had not clarified queries over possible undeclared nuclear activity.
Last week, the International Atomic Energy Agency voiced concern in a report to its members that Iran had not clarified the long-outstanding queries.
The regular report came at a delicate time when world powers were meeting to bring the US back to the Iran nuclear deal and Tehran back into compliance.
The 2015 accord has been hanging by a thread since the US left it in 2018 and reimposed sanctions, leading to Tehran to increase nuclear activities long curtailed by the deal.
Iranian ambassador Kazem Gharib Abadi told the IAEA's board of governors' meeting that the latest agency report is "not credible" and "deeply disappointing".
Iran insists it is co-operating with the IAEA as it seeks clarity on undeclared Iranian sites where nuclear activity may have taken place, mostly in the early 2000s.
"The secretariat has taken a counterproductive approach at the expense of its own credibility," Mr Abadi said.
He said this "could turn into an obstacle for future good-will interactions between the two sides".
IAEA Director General Rafael Grossi said on Monday that it was "becoming increasingly difficult" to extend a temporary inspections arrangement with Iran.
In February, Tehran suspended some IAEA inspections, leading the agency to negotiate continued but reduced access.
The latest understanding runs out on June 24.
A statement by France, Germany and the UK to the board of governors on Wednesday supported the IAEA's work.
"In limiting IAEA access, Iran makes it harder for the international community to assure themselves that Iran's activities remain exclusively peaceful," it said.
In a statement to the board, the US also urged Tehran to "fully co-operate".
Negotiations are expected to resume in two luxury Vienna hotels on Saturday to revive the 2015 accord, diplomats said.
Representatives from Britain, China, France, Germany, Russia and Iran, all party to the deal, have been meeting since early April.
Negotiators from the US are also taking part indirectly in the EU-chaired discussions.
Diplomats are hoping to conclude the negotiations before Iran's presidential election on June 18.