DUBAI, United Arab Emirates (AP) — Iran injected uranium gas into centrifuges at its underground Fordo nuclear complex early Thursday, taking its most-significant step away from its 2015 nuclear deal with world powers.

Tehran meanwhile also acknowledged blocking an official from the International Atomic Energy Agency from visiting its nuclear site at Natanz last week, the first known case of a United Nations inspector being blocked amid heightened tensions over its atomic program. Iran's representative to the IAEA said Tehran had asked the agency never to send the inspector again, without elaborating on what happened.

These latest steps by Iran put additional pressure on Europe to offer Tehran a way to sell its crude oil abroad despite the U.S. sanctions imposed on the country since President Donald Trump unilaterally withdrew America from the nuclear deal over a year ago.

The gas injection began after midnight at Fordo, a facility built under a mountain north of the Shiite holy city of Qom, the Atomic Energy Organization of Iran said. A U.N. official from the IAEA witnessed the injection, it said. The centrifuges ultimately will begin enriching uranium up to 4.5%, which is just beyond the limits of the nuclear deal, but nowhere near weapons-grade levels of 90%.

Fordo's 1,044 centrifuges previously spun without uranium gas for enrichment under the deal, which saw Iran limit its uranium enrichment in exchange for the lifting of economic sanctions. The deal had called for Fordo to become "a nuclear, physics and technology center."

Iran acknowledged Fordo's existence in 2009 amid a major pressure campaign by Western powers over Tehran's nuclear program. The West feared Iran could use its program to build a nuclear weapon; Iran insists the program is for peaceful purposes. Experts have suggested that the limits imposed under the 2015 deal, when obeyed, meant that Iran would need a year to gather enough material to build a nuclear weapon if it chose to do so — a time known as a "breakout period."

U.S. Secretary of State Mike Pompeo criticized Iran's decision to inject gas into the Fordo centrifuges in a statement Thursday. He made no reference to Trump's decision to leave the deal in May 2018, sparking the crisis.

"Iran's expansion of proliferation-sensitive activities raises concerns that Iran is positioning itself to have the option of a rapid nuclear breakout," Pompeo said. "It is now time for all nations to reject this regime's nuclear extortion and take serious steps to increase pressure."

Pompeo did not elaborate on what those serious steps should be. The U.S. earlier this week imposed sanctions on members of the inner circle of Iran's Supreme Leader Ayatollah Ali Khamenei.

Meanwhile, the Atomic Energy Organization of Iran said it had blocked a female IAEA inspector from its facility at Natanz, where centrifuges also enrich uranium. Iran said an alarm went off while the woman tried to enter the facility, causing officials there to stop her from going in.

The state-run IRNA news agency, citing Iran's atomic agency, said the woman was stopped "due to concerns over carrying suspicious materials." The inspector later left Iran without completing her visit, it said.

This marks the first known time of Iran blocking an inspector amid the tensions. Kazem Gharib Abadi, Iran's representative to the IAEA, described the breach the inspector allegedly caused a "serious security concern." He did not elaborate, other than to deny she had been detained.

"Due to the nature of the incident, Iran officially has asked the agency to remove her name from the list of designated inspectors for the Islamic Republic of Iran and to designate an alternative inspector instead," Abadi said.

Jackie Wolcott, the U.S. representative to the IAEA, called the inspector's rejection an "outrageous provocation."

"All board members need to make clear now and going forward that such actions are completely unacceptable, will not be tolerated, and must have consequences," Wolcott said in remarks released to journalists. "If the Iranian regime thinks it can test the international community's resolve on this issue, then we assure you the United States will not waver."

Iranian officials repeatedly have stressed the steps taken so far, including going beyond the deal's enrichment and stockpile limitations, could be reversed if Europe offers a way for it to avoid U.S. sanctions choking off its crude oil sales abroad. However, a European trade mechanism has yet to take hold and a French-proposed $15 billion line of credit has not emerged.

The collapse of the nuclear deal coincided with a tense summer of mysterious attacks on oil tankers and Saudi oil facilities that the U.S. blamed on Iran. Tehran denied the allegation, though it did seize oil tankers and shoot down a U.S. military surveillance drone.

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Associated Press writers Kiyoko Metzler and Philipp Jenne in Vienna and Matthew Lee in Washington contributed to this report.

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