FIRST ON FOX: The Iranian regime is closing in on the nuclear threshold, and the options available to the United States to stop Tehran from going nuclear are quickly narrowing, according to a new report released Thursday.

The report by the Washington D.C., based Foundation for Defense of Democracies (FDD) warns that the regime "is approaching the point at which no outside power could prevent it from building nuclear weapons."

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"As Tehran approaches that threshold, the United States will face an increasingly difficult choice between allowing the regime to cross over it or taking assertive measures – including potential military strikes – to stop Iran from going nuclear," the report by fellows Andrea Stricker and Anthony Ruggiero says.

The report comes the same day as the International Atomic Energy Agency (IAEA) said it believes Iran has increased significantly its stockpile of highly enriched uranium – in breach of the 2015 Iran nuclear deal, which both the U.S. and Iran subsequently left.

The Trump administration pulled the U.S. out of the deal in 2018 over concerns that it didn’t do enough to curb Iran’s nuclear ambitions in exchange for sanctions relief. The U.S. pulled out and subsequently slapped a number of sanctions on Iran, which, in turn, increased its nuclear activity.

Iran's Bushehr nuclear power plant

An Iranian flag flies at Iran's Bushehr nuclear power plant on Nov. 10, 2019. (Atta Kenare/AFP via Getty Images)

The Biden administration has since tried to re-enter the deal and talks are ongoing in Vienna with diplomats from Britain, China, France, Germany and Russia trying to bring Iran and the U.S. back into the deal.

The FDD report warns that, with the talks being drawn out by Iran, it is giving the regime more room to position itself to reach the nuclear threshold. Already it has enough uranium to produce weapons-grade uranium for at least four nukes and is expanding ballistic missile programs that it could use as delivery vehicles.

"At present, if the regime decides to make its first nuclear weapon, it may need as little as three weeks to produce enough fissile material," it says.

The authors also note the uncertainty from the incomplete intelligence reporting by the IAEA, which acts as the U.N.’s atomic watchdog. The Vienna-based agency said this week it has been unable to verify the exact size of Iran’s stockpile of enriched uranium due to limitations Tehran imposed on U.N. inspectors last year and that its monitoring and verification activities continue to be "seriously affected" by Iran's decision to stop letting inspectors access the agency's monitoring equipment.

The FDD report says the 2015 deal, known as the Joint Comprehensive Plan of Action (JCPOA), legitimized Iran’s advances toward the nuclear threshold, and while it prohibited weaponization, "the deal’s weak monitoring and verification provisions made this irrelevant."

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Since President Biden took office, the report notes, Iran has been increasing its enrichment of uranium toward weapons-grade purity, and says that should have provoked a "strong reaction" from the Biden administration and the IAEA – but it did not.

"By prolonging negotiations in Vienna, the Islamic Republic brought its breakout time close to zero while earning billions of dollars from oil exports thanks to Biden’s relaxation of sanctions as a goodwill gesture."

Now, the U.S. reportedly acknowledges that going back to the JCPOA’s "breakout" time of 7-12 months is not feasible, and it is more likely to be 6-9 months under a new deal – with the FDD report warning that an agreement that allows advanced centrifuges to remain would solidify that timeline.

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The authors state that, with a rapidly advancing Iran combined with uncertainty surrounding its intentions and activities, Washington’s options are narrow.

"Amid this uncertainty, President Biden might have to choose between carrying out military strikes based on incomplete or conflicting information or acquiescing to Iran’s development of nuclear weapons," the report says. "It would be preferable to keep Tehran far away from the threshold so that an American president never reaches this wrenching decision point."

It warns of a "flawed premise" shared by the Iran deal and the Biden administration that Iran can keep its uranium enrichment program while also being kept away from the nuclear threshold. 

Instead, the authors argue, the U.S. should move to restore an international consensus that Iran cannot be trusted with an enrichment program, and launch an economic, financial and political pressure program to force it back to the negotiating table – a program that includes restoring all prior sanctions on Iran.

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In the meantime, the report says the U.S. should consider the use of cyber-attacks and sabotage campaigns on nuclear sites to delay Iran’s progress.

"The Biden administration should take all related measures necessary to ensure that the world’s most prolific state sponsor of terrorism can never reach the nuclear threshold," they argue.

Fox News’ Ben Evansky and The Associated Press contributed to this report.