The International Atomic Energy Agency (IAEA) says Iran is blocking access to some of its nuclear sites and continues to boost its stocks of uranium enriched above the percentage allowed in its hobbled 2015 deal with world powers.
The IAEA said in a confidential report cited by Western news agencies on September 7 that Iran had also failed to answer questions including on uranium traces found at three undeclared sites.
"The director-general is increasingly concerned that even after some two years the safeguards issues outlined above in relation to the four locations in Iran not declared to the agency remain unresolved," the IAEA said in one of two quarterly reports on Iran.
Under the nuclear deal, Iran was not meant to enrich uranium above 3.67 percent, well below the 90 percent threshold needed for use in a nuclear weapon.
However, the report estimates that Iran now has 84.3 kilograms of uranium enriched to 20 percent, up from 62.8 kilograms when the IAEA last reported in May; as well as 10 kilograms enriched up to 60 percent (up from 2.4 kilograms).
The confidential reports by IAEA Director-General Rafael Grossi to IAEA member states are being issued ahead of next week's meeting of its 35-country board of governors.
The reports come as indirect talks between Tehran and Washington aimed at reviving the 2015 nuclear deal have stalled amid the change of government in Iran.
Six rounds of talks on reviving the accord were held in Vienna between April and June to bring Tehran and Washington back into compliance.
Former U.S. President Donald Trump withdrew from the nuclear accord in 2018 and reimposed tough sanctions that have crippled Iran's economy.
Iran responded by gradually decreasing its commitments under the deal and expanding its nuclear work.
Earlier this week, Iran's Foreign Ministry said Tehran will "definitely" continue the nuclear talks in Vienna, without specifying when the country will be ready to resume negotiations.
Under the 2015 deal between Iran and Britain, France, Germany, China, Russia, and the United States, Tehran agreed to curb its nuclear activities in exchange for sanctions relief.