The top inspector for the UN’s nuclear agency has reportedly accused Iran of evading attempts to gather information on Tehran’s uranium production at a warehouse that was flagged by Israel and where particles were found earlier this year.

Bloomberg, in a report Thursday, cited two officials as saying that Massimo Aparo told diplomats in a closed-door meeting in Vienna on Wednesday that the Islamic Republic was “evading attempts to discover the source of manmade and natural uranium particles detected at a warehouse in Tehran.”

The extraordinary meeting of the International Atomic Energy Agency’s board of governors was convened by Cornel Feruta, the organization’s acting head, to discuss the latest concerns about Iran’s nuclear program, the report said.

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In September, Reuters reported that traces of uranium were found at a facility in Tehran that was alleged by Prime Minister Benjamin Netanyahu to be a “secret atomic warehouse.”

Iran has not provided an explanation for why uranium was found at the site to the IAEA, which is investigating the facility in the Iranian capital, the news agency reported at the time.

In a speech last year at the UN General Assembly, Netanyahu revealed the existence of the warehouse in Tehran, which he said held “massive amounts” of equipment and material that were part of a secret Iranian nuclear program.

Netanyahu called for the IAEA to inspect the facility and, in July, Israeli television reported that soil samples from the warehouse turned up “traces of radioactive material,” without specifying the type.

Citing two unnamed diplomats, Reuters reported that the material found at the site was determined to be uranium. One of the diplomats, however, said the uranium was not enriched enough to be used for a nuclear bomb.

“There are lots of possible explanations” for why uranium traces were found there, the diplomat said.

The report said it was difficult to determine the origin of the uranium since Iran has not provided answers. It noted it was unclear whether the traces came from material at the site before or after the 2015 international deal was signed to limit Tehran’s nuclear program.

Iran’s failure to explain why uranium was found at the warehouse has fueled tensions with the United States, according to Reuters.

US President Donald Trump pulled out of the nuclear accord last year and reimposed stringent sanctions on Iran. Tehran has subsequently taken a number of steps to roll back its commitment to the accord over what it says is a failure by the deal’s remaining signatories to provide economic relief from the sanctions.

On Wednesday, Iran began injecting centrifuges with uranium at the Fordo facility, where it was barred from conducting such work under the nuclear deal.

Iran’s announcement that it would resume enrichment at the Fordo plant from midnight drew a chorus of concern from the remaining parties to the troubled agreement and the US called for “serious steps” to be taken, warning that Tehran was “positioning itself to have the option of a rapid nuclear breakout.”

Also Thursday, Iran said it had canceled the the accreditation of a UN nuclear inspector after she triggered an alarm last week at the entrance to the Natanz uranium enrichment plant. The EU said it was “deeply concerned” and called on Iran to ensure no further such incidents occur.

In September, the Wall Street Journal reported that Iran was obstructing the investigation into the Tehran site identified by Netanyahu. Unidentified diplomats told the newspaper Iran was refusing to provide answers to questions posed by the IAEA, in what was apparently the first instance of Tehran failing to cooperate with inspectors.

The diplomats told the WSJ that the uranium traces were likely remains from Iran’s past experimentation in nuclear weapons development. Iran has denied ever seeking nuclear weapons, though Israeli and Western intelligence strongly dispute those assertions. The diplomats said the material’s existence at the site was unlikely to indicate new work on weapons development, but would be a breach of Iran’s commitment to non-proliferation.

Iran has denied that the site was a nuclear facility or served any secretive purpose. In an initial response to Netanyahu’s UN speech, Iranian state media claimed the warehouse was actually a recycling facility for scrap metal.

Agencies contributed to this report.