Berlin — The United Nations' atomic watchdog confirmed Monday that Iran has moved ahead with preparations to use more advanced centrifuges, in breach of limitations set by the unraveling 2015 nuclear deal with major powers. The International Atomic Energy Agency said its inspectors verified new centrifuges, including 30 advanced IR-6 and three IR-6s models, had been or were being installed but not yet tested.

The announcement followed the return from Tehran Sunday of IAEA Acting Director General Cornel Feruta, who was expected to brief the agency's board of governors later on Monday after his trip.

He was also expected to shed more light on a report by the Reuters news agency that IAEA inspectors had discovered trace amounts of uranium at a facility in Tehran, flagged by Israel last year as a "secret atomic warehouse," which Iran has thus far not explained.

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Uranium traces at Tehran facility

The 2015 nuclear deal demanded a complete accounting by Iran of all its previous atomic work, and oversight of all ongoing work. The nuclear work at this facility appeared to fall outside of the details handed over by Tehran as part of the agreement.

It was not immediately clear, however — and has not been since Israeli leader Benjamin Netanyahu first touted the facility as evidence Iran tried to conceal a nuclear weapons program — whether the materials at the Tehran facility predated the 2015 accord or showed any violation of it. Iran denies it has ever sought to develop nuclear weapons.

IAEA Spokesman Fredrik Dahl told CBS News' Pamela Falk that the Agency's visit to Iran over the weekend was part of, "ongoing interactions between the IAEA and Iran related to the implementation of the Safeguards Agreement and Additional Protocol, and the IAEA's verification and monitoring in Iran under the Joint Comprehensive Plan of Action (the nuclear deal)."

Falk said the Director General's report to the Board was to lay out everything the inspectors discovered during their visit and in their discussions with Iranian officials.

The prepared opening statement from the IAEA chief to the board appeared to acknowledge the ongoing failure by Iran to explain the traces of uranium detected at the Tehran facility.

Feruta was to tell the board that the IAEA "continues to verify the non-diversion of nuclear material declared by Iran under its Safeguards Agreement. Evaluations regarding the absence of undeclared nuclear material and activities in Iran continue."

He said he had emphasized to Iranian officials "the importance of full and timely cooperation by Iran... I also stressed the need for Iran to respond promptly to Agency questions related to the completeness of Iran's safeguards declarations. The Agency will continue its efforts and will remain actively engaged. Time is of the essence."

Proof of Israel's allegations?

Even before the report, Israel and the U.S. were quick to use the new information to heap more pressure on Iran.

President Trump's National Security Adviser, a long-time hawk on Iran, alluded to the revelation that uranium was found at the facility in a tweet over the weekend, saying it proved Iran "may be concealing nuclear material and/or activities." He said the U.S. and other IAEA member states were "eager to get a full report as soon as possible."

IAEA Acting Director General going to Iran just as IAEA informs its Board that #Iran may be concealing nuclear material and/or activities. We join with other @iaeaorg Board member states eager to get a full report as soon as possible. — John Bolton (@AmbJohnBolton) September 7, 2019

Israel's U.N. Ambassador Danny Danon claimed the IAEA appeared to be verifying Israel's initial assertion about the Tehran site, telling Falk before the meeting on Monday that the agency's inspectors showed there were enrichment activities at the facility that were not reported as required.

Israel and the Trump administration have long criticized the Iran nuclear deal as too generous to Tehran, arguing that it allowed the Islamic Republic to continue other nefarious activities, including its conventional missile program and support for proxy groups across the Middle East, unchecked.

Maximum pressure and rising tension

Iran was infuriated by the Trump administration's unilateral decision to walk away from the deal signed by former President Barack Obama and the subsequent "maximum pressure" campaign of sanctions imposed by Mr. Trump. Iran calls the sanctions "economic warfare," and says the impact is primarily on regular Iranians.

The White House insists the sanctions are meant to force the Iranian regime back to the table to renegotiate the terms of the nuclear deal.

But Tehran has refused to re-open negotiations with the U.S., and has instead pushed the other parties to the 2015 agreement, mainly Europe, to find a way to allow Iran to keep selling its crude oil abroad — essentially to evade the U.S. sanctions. Europe failed to do that before a deadline set by Tehran last Friday, leading to the latest breach by Iran of the nuclear deal's terms.

A centrifuge enriches uranium by spinning uranium hexafluoride gas. Under the atomic accord, Iran is limited to operating 5,060 older IR-1 centrifuges. Bringing the more advanced centrifuges online would give the Iranians' the ability to enrich more uranium, and to a higher degree of purity. Iran has warned that if the nuclear deal continues to unravel, it will do just that.

Right now it is limited to enriching uranium to less than 5% purity, which makes fissile material usable for medical and research purposes. If they refine to 20%, a level they have yet to reach, it would be a short technological step to the purity required to manufacture a nuclear warhead.

Both the U.S. and Israel have vowed that Iran will never be allowed to gain that capacity. As the war of words between Iran and the U.S. continues, the Islamic Republic has also shown it's ire at the U.S. withdrawal from the agreement by interfering in shipping through the Strait of Hormuz, and with increased attacks by proxy groups in Lebanon and Iraq.