"I am deeply concerned that nuclear material has been present at the three undeclared locations in Iran and that the current locations of this nuclear material are not known by the agency," he said.
Grossi told the International Atomic Energy Agency’s board of directors that Iran’s written statement on the fourth site came without substantiating documentation.
He said that the presence of man-made uranium particles at these locations, including of isotopically altered particles, proved that nuclear material or equipment contaminated by it had been present there.
"I reiterate the requirement for Iran to clarify and resolve these issues without further delay by providing information, documentation and answers to the Agency’s questions," Grossi stressed.
Iran's refusal to give the agency clarity on the nuclear material present seriously affects the agency's ability to guarantee the peaceful nature of Iran's nuclear program, he added.
Back in May, the UN watchdog released a report suggesting Iran has failed to explain traces of uranium found at several undeclared sites.
"After many months, Iran has not provided the necessary explanation for the presence of the nuclear material particles at any of the three locations where the Agency has conducted complementary accesses (inspections). The Director General is concerned that the technical discussions between the Agency and Iran have not yielded the expected results. The lack of progress in clarifying the Agency's questions concerning the correctness and completeness of Iran's safeguards declarations seriously affects the ability of the Agency to provide assurance of the peaceful nature of Iran's nuclear programme," a report by Grossi said.
At Iran's main enrichment plant, which is underground at Natanz, the IAEA verified on 24 May that 20 cascades, or clusters, of different types of centrifuges were being fed with uranium hexafluoride feedstock for enrichment.
After Washington pulled out of the nuclear deal in 2018 under President Donald Trump and re-imposed crippling economic sanctions against Tehran, Iran began scaling back its commitments under the agreement.
According to the IAEA, Iran's stock of enriched uranium is estimated to be some 3,241 kg as of 22 May. This far exceeds the 202.8 kg limit set by the JCPOA agreement.