Uranium of a man-made origin has been discovered in Iran at a site that has "not been declared" to inspectors, the United Nation's nuclear watchdog disclosed for the first time in a report Monday.

The explosive revelation comes amid other reports Tehran has also been enriching uranium at an underground facility.

The International Atomic Energy Agency said in its confidential quarterly report distributed to member states, and seen by news agencies such as Reuters and the Associated Press, that Iran has begun enriching uranium at its underground Fordo facility, a move prohibited by the 2015 nuclear deal.

Iran's stockpile of low-enriched uranium still exceeds the amount allowed by the Joint Comprehensive Plan of Action, as the country continues to enrich uranium up to 4.5 percent, above the 3.67 percent allowed under the 2015 deal.

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The IAEA said that in addition to the underground Fordow site, uranium of a man-made origin has been discovered "at a location in Iran not declared to the agency."

The disclosure was the first time the watchdog acknowledged in the confidential report that allegations made by the U.S. and Israel against the Islamic Republic are true. The IAEA did not identify the site, but Israel and the U.S. have previously said the location was on the outskirts of Tehran.

Israeli Prime Minister Benjamin Netanyahu has previously described the location as a "secret atomic warehouse." Israel has alleged that material at the site comes from an Iranian military program involving work on nuclear weapons, while Iran denies pursuing nuclear weapons and says its program is peaceful.

While Iran has claimed the program is for peace, it's uranium stock continues to grow.

Iran invited inspectors from the IAEA to Fordo to see its work, where they found as of Nov. 3, the country’s stockpile of low-enriched uranium was 372.3 kilograms (820.78 pounds) compared to 241.6 kilograms reported on Aug. 19, and past the 202.8-kilogram limit.

The 2015 nuclear deal had called for Fordo to become a research center, but it is now home to more than 1,000 centrifuges.

Iran is also enriching uranium with centrifuge machines other than its most basic model, the IR-1, which is not allowed under the deal, according to Reuters. The report reviewed by Reuters showed that Iran has enriched with more advanced centrifuges and installed small numbers of centrifuges not mentioned in the deal.

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Since the U.S. pulled out of the Iran nuclear pact last year and imposed sanctions, Iran has been slowly violating its provisions to pressure other nations to provide more incentives.

European Union foreign ministers on Monday affirmed their support for the nuclear deal with Iran, even though the Islamic Republic continues to defy it.

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EU foreign policy chief Federica Mogherini said that the ministers underlined their "full commitment to the agreement that remains crucial for our security, even if it's increasingly difficult to preserve it. We will continue our efforts to have a full implementation of the agreement."

The EU sees the nuclear pact as a key component of regional and global security and has struggled to stop it from unraveling since President Trump unilaterally pulled the United States out of it over a year ago, triggering debilitating economic sanctions against Iran.

The Associated Press contributed to this report.