ISTANBUL - Iran has prevented a United Nations nuclear inspector from entering a uranium-enrichment facility and revoked her credentials after Iranian officials said she tested positive for explosive nitrates.

The move drew condemnation from the United States, which called Iran's decision to expel the inspector an "outrageous provocation."

A spokesman for the International Atomic Energy Agency, the U.N. nuclear watchdog, confirmed that one of its inspectors was "temporarily prevented from leaving Iran" last week.

"The Agency does not agree with Iran's characterization of the situation involving the inspector, who was carrying out official safeguard duties in Iran," said Fredrik Dahl, the agency's spokesman. "Preventing an inspector from leaving a country, particularly when instructed to do so by the Agency, is not acceptable and should not occur," the agency's acting director general, Cornel Feruta, said in a statement.

According to Iranian officials, the inspector was halted by security at the gate of Iran's main enrichment plant in Natanz after triggering an alarm. The alert raised officials' concerns she was carrying "suspicious material."

The Atomic Energy Organization of Iran said Wednesday that her credentials were revoked and that the IAEA was informed of the incident. Iran did not immediately provide further details about the inspector, including her nationality.

In remarks Thursday in Vienna, where the IAEA is headquartered, Tehran's envoy to the agency said the inspector, who arrived with a team, had initially tested positive for explosive nitrates. The other members were cleared to enter.

"We repeated the [security] process with different detectors and also her suitcase, and the alarms went off every time," Kazem Gharbabadi, Iran's ambassador to the IAEA, said in an interview Thursday with Iranian state television. He said the woman went to the bathroom at one point while awaiting a further screening and apparently removed the material. He did not offer an explanation for why the inspector allegedly had the residue on her, but he indicated it was deemed a threat.

"Considering the history of sabotage against our nuclear facilities, we are not going to tolerate risking our national security," Gharbabadi said. "We have asked the agency to replace this inspector with someone else."

The United States and Iran have been facing off over Tehran's nuclear program, after President Trump withdrew from a 2015 agreement that Iran reached with world powers curbing Iran's atomic activities in exchange for the lifting of sanctions. The Trump administration reimposed a harsh embargo on the Iranian economy last year.

In a bid to compel the deal's other signatories - including European nations - to offset the effects of U.S. sanctions, Iran has deliberately breached several of the landmark accord's restrictions.

In recent months, Tehran has exceeded limitations on the size and purity of its enriched-uranium stockpile. And earlier this week, Iran began injecting uranium gas into centrifuges at the Fordow enrichment facility near the city of Qom. Under the nuclear deal's provisions, enrichment activities are banned at the site.

On Thursday, the IAEA convened a board of governors meeting to discuss Iran's nuclear-related commitments under the agreement, the agency said. The IAEA verified what it said was the transfer of a cylinder of uranium gas to Fordow to be connected to Iran's IR-1 centrifuges, according to Dahl, the spokesman.

It provided no further details, however, about the issues discussed at Thursday's meeting.

The U.S. envoy to the agency, Jackie Wolcott, said in a statement that the IAEA's report Thursday indicated that Iran "is not cooperating adequately on crucial aspects of its safeguards obligations with the agency." In particular, she cited concerns about "particles detected by the IAEA."

The IAEA reportedly found suspicious samples at the Turquz Abad site near Tehran, a location raided by Israeli intelligence agents in early 2018. Israeli agents had recovered a trove of documents on Iran's nuclear effort and discovered the presence of processed nuclear material that Tehran had never declared to the monitoring group, Israeli officials said.

"Iran has refused to provide . . . a credible, verifiable answer to the fundamental question of where the particles detected by the IAEA came from," Wolcott said.

"This is a significant new revelation that raises questions that lie at the core of Iran's safeguards obligations," she continued. "Has Iran declared to the Agency all of its reportable nuclear material, sites and facilities? Or is it continuing to conceal sensitive activities from the IAEA?"

According to the IAEA, Iran continues to enrich uranium at levels far below the 90 percent threshold required for a nuclear weapon.

Iran's atomic energy commission said the Fordow facility would be enriching uranium only up to 4.5% by Saturday, a level sufficient for fuel to power energy-producing nuclear reactors but well below weapons grade.

"While the step does not pose a near-term proliferation threat, it risks eroding European support, and closing the window on negotiations to restore full compliance with the agreement," the Washington-based Arms Control Association, a public policy organization, said in a statement about Iran's move to reactivate the Fordow facility.

Following the IAEA meeting Thursday, Israeli officials, speaking on background, cited reports that the agency had addressed the concerns at Turquz Abad and said this vindicated Israel's campaign to reveal Iran as a persistent nuclear cheat.

In the months since Israel's intelligence raid at Turquz Abad, senior Israeli security officials said Iran had scrubbed the site and removed suspected nuclear materials.

The officials, who briefed reporters in Jerusalem, said the long-suspected head of Iran's nuclear program, Mohsen Fakhrizadeh, was also in charge of the site at Turquz Abad.

"This work continues at the highest levels of government," one official said.

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Hendrix reported from Jerusalem. The Washington Post's Michael Birnbaum contributed to this report.