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Iran could now produce “one bomb’s worth of fissile material” in “about 12 days,” a top US Defense Department official said Tuesday.

Under Secretary of Defense for Policy Colin Kahl offered one of the most striking US government assessments to date of Iran’s “breakout time” as efforts to try to restore the Iran nuclear agreement remain halted and Tehran continues to breach the restrictions set out by the deal.

Kahl said that “Iran’s nuclear progress since” the Trump administration withdrew the United States from the 2015 nuclear deal “has been remarkable.”

“Back in 2018, when the previous administration decided to leave the JCPOA, it would have taken Iran about 12 months to produce one fissile, one bomb’s worth of fissile material,” Kahl said at a House Armed Services Committee hearing, using the acronym for the formal name of the deal: the Joint Comprehensive Plan of Action.

“Now it would take about 12 days,” he said.

Biden administration officials have for months said that Iran’s breakout time – the amount of time it would need to produce enough weapons-grade material for one nuclear bomb – had dwindled to a matter of weeks. The breakout time does not mean that Iran could produce an actual bomb in that amount of time.

Kahl’s comments – set against reports of Iran’s continued enrichment of uranium in violation of the deal – present a striking picture of the challenges facing the increasingly defunct agreement.

In a restricted report seen by CNN, the International Atomic Energy Agency confirmed that uranium particles enriched to 83.7% purity – which is close to the 90% enrichment levels needed to make a nuclear bomb – had been found in Iran’s Fordow Fuel Enrichment Plant, an underground nuclear facility.

According to the confidential IAEA report, Iran’s stockpile of uranium enriched up to 60% had grown by 25.2 kg to 87.5 kg since the agency’s last quarterly report.

“It is quite clear that Iran enriched uranium to near weapons-grade levels,” Kelsey Davenport, the Director for Nonproliferation Policy at the Arms Control Association, told CNN.

She said Tehran may have been “testing political responses to higher levels of enrichment or it was experimenting and it got caught,” noting that it underscores the rapid advancement of Iran’s nuclear program and the high risk of miscalculation.

Davenport also told CNN that Iran’s “significant increase” in its enriched uranium stockpiles is “a serious concern.”

“When Iran can rapidly produce enough weapons grade material for several bombs, the proliferation risk rises significantly,” she explained.

“This is an urgent proliferation crisis,” she said.

US officials have repeatedly said they are seeking a diplomatic solution to the matter, but have maintained that all options, including military ones, are on the table.

More than a year of indirect negotiations between the US and Iran to try to restore the deal broke down in September 2022, as the US accused Iran of making “unreasonable” demands related to a probe by the IAEA, which is the UN’s nuclear watchdog, into unexplained traces of uranium found at undisclosed Iranian sites.

In recent months, the administration has said the agreement is “not on the agenda,” in the words of State Department spokesperson Ned Price.

Kahl said Tuesday that the agreement is “on ice” in part because “Iran’s behavior has changed since then, not the least of which their support for Russia and Ukraine.”

This story has been updated with additional details.

CNN’s Michael Conte contributed to this report.