Last week, the International Atomic Energy Agency (IAEA) voiced concern in a report to its members that Iran had not clarified the long-outstanding queries.

The regular report comes at a delicate time when world powers are meeting to bring the US back to the Iran nuclear deal and Tehran back into compliance with it.

The 2015 landmark accord has been hanging by a thread since the US left it in 2018 and re-imposed sanctions, leading to Tehran to step up its nuclear activities long curtailed by the deal.

Iran Ambassador Kazem Gharib Abadi told the IAEA's board of governors' meeting that the latest agency report is "not credible", calling it "deeply disappointing".

Iran insists it is cooperating with the IAEA as it seeks clarity on several 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," he said, warning this "could turn into an obstacle for future good-will interactions between the two sides".

IAEA Director General Rafael Grossi told reporters on Monday that it was "becoming increasingly difficult" to extend a temporary inspections arrangement with Iran.

In February, Tehran suspended some IAEA inspections, leading to the agency to negotiate continued though reduced access.

The latest understanding runs out on June 24.

A joint statement by France, Germany and the UK to the board of governors 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.

The US also urged Iran in a statement to the board to "fully cooperate".

Negotiators are expected to resume their talks in two downtown Vienna luxury hotels on Saturday to revive the 2015 accord, according to diplomats.

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.