The head of the UN's nuclear watchdog said on Tuesday he was "extremely concerned" about what he described as Iran's lack of co-operation about unexplained traces of uranium in the country.
Rafael Grossi, the head of the International Atomic Energy Agency, said Iran "has not been forthcoming" with information his organisation is trying to extract.
The two sides had struck an agreement in March with the aim of concluding the IAEA's inquiries about uranium traces at old but undeclared sites in Iran.
But Mr Grossi told the European Parliament on Tuesday that IAEA operatives "in the last few months were able to identify traces of enriched uranium, in places that had never been declared by Iran as places where any activity was taking place".
"The situation does not look very good," he said. "Iran, for the time being, has not been forthcoming in the kind of information we need from them ... we are extremely concerned about this."
The IAEA's talks with Iran are separate from negotiations between world powers aimed at restoring the 2015 nuclear deal, which the US abandoned four years ago. But both sides have indicated that ironing out the first issue could smooth progress towards the second.
The talks on the nuclear deal, formally known as the Joint Comprehensive Plan of Action, broke off in March after diplomats had described an agreement as being close. Russia's involvement in the talks added further complexity after its invasion of Ukraine poisoned the atmosphere between Moscow and the West.
A potential revival of the 2015 deal would see sanctions lifted on Iran if it agrees not to breach limits set on its atomic activity, which are designed to keep nuclear weapons out of its hands but which it has openly flouted since the US quit the pact.
Diplomats have said the window for agreement is closing because Iran's nuclear progress will eventually render a deal to curb its activities obsolete. Tehran announced on Monday that European Union diplomat Enrique Mora, who chairs the negotiations that had been taking place in Vienna, would visit Tehran this week.
Mr Grossi said those talks were "at a sort of pause" but said his agency was "of course still hopeful that some agreement is going to be reached within a reasonable timeframe".
However, the IAEA was "trying to clarify a number of still-open matters with Iran", said Mr Grossi, who has said his watchdog will not drop its investigations to help seal a JCPOA deal. Documents are to be exchanged between the two sides by May 22.
Iran "should be at the top of our preoccupations in spite of the drama that is unfolding in Ukraine", Mr Grossi said, after fighting near nuclear sites, including the Chernobyl exclusion zone, led to fears of radioactive fallout.