Traces of uranium were found in samples taken by United Nations nuclear inspectors from a Tehran facility alleged by Prime Minister Benjamin Netanyahu to be a “secret atomic warehouse,” according to a report Sunday.

Iran has not provided an explanation for why uranium was found at the site to the UN’s International Atomic Energy Agency, which is investigating the facility in the Iranian capital, Reuters reported.

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.

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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 IAEA has been seeking answers from Tehran for two months, a senior diplomat said, with no success.

“It is not something that is… unique to Iran. The agency has these cases in many other situations,” the diplomat was quoted as saying. “Depending on the engagement it can take two months, six months.”

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. Over the weekend, Iran said it would activate advanced centrifuges that enrich uranium at a faster rate.

Despite the latest move, the UN agency said Iran would allow the IAEA to continue monitoring its nuclear facilities in accordance with the 2015 agreement.

The Reuters report came as Cornel Feruta, the acting head of the IAEA, was in Tehran for talks with Iranian officials.

His visit to Tehran comes a day before the IAEA board of governors convenes for a quarterly meeting in Vienna, at which its verification and monitoring mission in Iran will be discussed.

Quoting unnamed Israeli sources, Channel 13 reported Sunday that a report set to be presented at the meeting is highly critical of Iran. The network said the United States and Israel are pushing for the public release of the report, but this is opposed by unspecified countries.

Last week, 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 said there were internal disagreements in the IAEA on how severely Iran should be censured over the issue. A recent IAEA report on Iran’s growing breach of the 2015 nuclear deal reportedly made only vague reference to Tehran’s lack of cooperation with inspectors, saying “Ongoing interactions between the Agency and Iran relating to Iran’s implementation of its Safeguards Agreement and Additional Protocol require full and timely cooperation by Iran.”

The diplomats told the WSJ that the 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.

In that speech, Netanyahu claimed some 15 kilograms (33 pounds) of radioactive material had been recently removed from the atomic warehouse and squirreled away around Tehran, endangering the capital’s residents. The site may have contained as much as 300 tons of nuclear-related equipment and material in 15 shipping containers, Netanyahu added. He did not specify what nuclear material was contained at the site.

That speech came months after Israel’s disclosure that it had spirited away what it said was a “half-ton” of Iranian nuclear documents from Tehran, with Netanyahu saying both the archive and the warehouse were proof that Iran continues to seek atomic weapons despite the 2015 nuclear deal. “Iran has not abandoned its goal to develop nuclear weapons…. Rest assured that will not happen. What Iran hides, Israel will find,” Netanyahu told the UN.

Agencies contributed to this report.