The International Atomic Energy Agency says Iran is continuing to enrich uranium to 60 percent purity, breaching a 2015 nuclear deal between the country and the world's major powers.
The UN nuclear watchdog compiled its latest report on Iran's nuclear development on Monday.
The report said as of May 22, Iran had produced 2.4 kilograms of 60-percent enriched uranium. The level is far higher than the 3.67 percent purity limit set under the deal.
It also said the agency hasn't been able to access data for monitoring Iran's nuclear program since February, when the Islamic republic started limiting international inspectors' access to its nuclear sites.
Tehran cited at the time the refusal of the United States to lift sanctions.
The agency expressed grave concern in another report about Iran's failure to explain traces of a nuclear substance found at multiple facilities in the country.
The United States and Iran have engaged in indirect talks in Vienna about reviving the 2015 accord.
But negotiations have stalled as differences remain between the two sides.
The IAEA's board of governors will hold a session next Monday to discuss issues including Iran's violation of the nuclear deal.