U.S. State Department officials described Iran's blocking of an international nuclear inspector from accessing key nuclear sites last week as an "outrageous and unwarranted act of intimidation" amid growing concerns Iran is hiding undeclared nuclear materials.

Iran confirmed earlier this week it had blocked an international nuclear inspector from accessing key sites for mandated oversight.

U.S. officials and nuclear experts described Iran's behavior as highly unusual and dangerous. As the country continues to bolster its enrichment of uranium—the key component in a nuclear weapon—it is becoming increasingly clear that Tehran has no interest in living up to international commitments to provide transparency about its nuclear program, particularly regarding work on the weapons front.

Iran said that it had blocked at least one inspector from physically entering its Natanz uranium enrichment site because she was carrying "suspicious" materials that could have been explosive. However, nuclear inspectors commonly carry signs of explosive residue due to the nature of their work, and safety procedures for such situations exist, experts told the Washington Free Beacon.

A State Department spokesman confirmed to the Free Beacon that Iran had detained the IAEA inspector, adding that the incident bolsters concerns the Trump administration has about Iran being in possession of undeclared nuclear materials.

"Iran's detention of an IAEA inspector was without question an outrageous and unwarranted act of intimidation," the State Department official said on background. "Iran's justifications strain credibility—IAEA inspectors must be permitted to conduct their work unimpeded. Although the inspector is now safe, such acts by Iran will not be tolerated."

The administration suspects that Iran is trying to prevent international inspectors from confirming its work with prohibited nuclear materials.

"The United States is deeply concerned about the two issues the IAEA acting director general described in today's special session of the IAEA Board of Directors," the official said. "First, that the IAEA has detected evidence of potential undeclared nuclear material in Iran, and second, the detention of an IAEA inspector. Along with Iran's expansion of proliferation-sensitive nuclear activity, this pattern of deception and intimidation is unacceptable. All nations should be concerned that Iran is not fully cooperating with the IAEA and should demand Iran immediately redress these serious problems."

The diplomatic escalation comes as Iran breaches limits on the amount of enriched uranium it produces and the enrichment methods it uses. It escalated installations of advanced centrifuges in the past week and has vowed to continue doing so.

Nuclear experts told the Free Beacon that Iran's behavior raises multiple questions and concerns about the nature of its ongoing work.

"Assuming the IAEA version of events is correct and she did not have explosive contamination on her person, then Iran may be testing what the reaction is to denying inspectors access to safeguarded sites," David Albright, a former weapons inspector and president of the Institute for Science and International Security, told the Free Beacon.

"How long does it take for this episode to be reported to the board and media?" he asked. "Does the IAEA send a replacement quickly? How many countries and which ones believe the Iranian rationale? Is there outrage or are there divisions that delay a coordinated response?"

These questions must be addressed by the international community, Albright said.

"If Iran wants to break out and needs to do certain banned things at a safeguarded site to accomplish that breakout, it will need to deny inspectors access to that site for some period of time," he said. "Although the following is just speculation at this point, Iran may be testing the waters and scoping out the best way to deny access in the future when it really needs to do it."

Andrea Stricker, a nonproliferation analyst and research fellow at the Foundation for Defense of Democracies, described Iran's actions as "highly provocative."

It "gives the impression that Iran could be considering curtailing inspection authorities as a future step to draw down its JCPOA commitments," Stricker said. "It's a hostile sign for sure."

While Iranian officials claimed they barred the IAEA inspector due to "suspicious" materials being detected on her person, experts say traces of explosive materials should not have necessarily raised alarms following an investigation.

Stricker said IAEA inspectors can carry traces of explosive materials, but procedures are typically in place to address concerns like those Iran expressed.

"It is not uncommon for inspectors to pick up explosive residue if they have been at a high explosives site," she said. "They can be detained at airports, for example. However, they have procedures in place to remedy such situations. I think that given the seriousness with which countries have taken this, there is more to it and it is worth looking into."