The UN's International Atomic Energy Agency (IAEA) said that Iran has increased its production of enriched uranium in recent months, according to a report seen by multiple news agencies on Saturday.
The news comes amid talks between Iran and the US aimed at curtailing Tehran's nuclear program and preventing it from acquiring nuclear weapons.
Does Iran have enough enriched uranium to build a bomb?
The IAEA quarterly report expressed "serious concern" over the estimated 408.6 kilograms (around 900 lbs) enriched to 60% as of May 17, marking an increase of 133.8 kilograms since the previous report in February.
In theory, approximately 42 kilograms of 60% enriched uranium is enough to produce one atomic bomb if it is further enriched to 90%.
IAEA chief Matteo Grossi has previously said that Iran could make several nuclear bombs if it chose to do so.
Iran has repeatedly denied any intention of acquiring nuclear weapons, but the IAEA pointed out that no other non-nuclear state has enriched uranium to 60%.
IAEA says Iran's cooperation 'less than satisfactory'
The nuclear watchdog also criticized Iran for its "less than satisfactory" cooperation in a separate in-depth report.
"While Iran continues to cooperate with the agency on matters of routine safeguards implementation, in a number of respects... its cooperation with the agency has been less than satisfactory," the report said.
"In particular, Iran has repeatedly either not answered, or not provided technically credible answers to, the agency's questions and has sanitised locations as listed in this report, which has impeded agency verification activities," it added.
Grossi said Saturday that he "reiterates his urgent call upon Iran to cooperate fully and effectively" with the IAEA's ongoing investigation.
Why is the US seeking a nuclear deal with Iran?
Iran's nuclear program has been a major focus of US foreign policy for years.
The two sides reached an agreement in 2015 that saw the easing of US sanctions in exchange for limits to Iran's nuclear program.
This deal was torpedoed by President Donald Trump in his first term when he unilaterally withdrew from it.
Trump's second term has seen a renewed interest in finding a deal with Iran, causing tension with Iran's archrival Israel — itself believed to be a nuclear power.
Israeli Prime Minister Benjamin Netanyahu's office was quick to respond to the IAEA report on Sunday, saying it was a clear warning sign that "Iran is totally determined to complete its nuclear weapons program."