National Security Advisor Jake Sullivan took an ominous tone Friday when speaking about efforts to bring Iran back in line with a 2015 nuclear agreement saying, "It's not going well."

"We do not yet have a pathway back into the JCPOA," he said, referring to the Joint Comprehensive Plan of Action.

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President Hassan Rouhani, second right, listens to head of the Atomic Energy Organization of Iran Ali Akbar Salehi while visiting an exhibition of Iran's new nuclear achievements in Tehran, Iran, in April.

Sullivan said there has been "some progress" over the last few days but did not appear hopeful that a concrete resolution would be found following months of indirect talks with Tehran.

President Biden made it a top priority for his administration to bring Iran back to the negotiating table and re-enter the JCPOA, which fell apart after the Trump administration removed the U.S. from the agreement in 2018.

While European allies warned the move could weaken the nuclear agreement, the Trump administration maintained its withdrawal did not void Iran’s part of the bargain. 

But by the end of Trump's time in office, Iran began blocking access to international inspections of its nuclear facilities and continued developing its nuclear capabilities. 

"Since we walked away from a deal that had fundamentally put a lid on Iran's nuclear program, they have raced that program forward," Sullivan said Friday, speaking from an event hosted by the Council on Foreign Relations.

The election of anti-Western President Ebrahim Raisi in June only further stonewalled Western efforts to negotiate with Tehran, and Iran has repeatedly paused talks with the other signatories of the JCPOA.

Iranian President Ebrahim Raisi addresses the parliament during a vote of confidence session for the education minister, in Tehran, Iran. (AP Photo/Vahid Salemi)

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On Thursday, Iran’s chief negotiator, Ali Bagheri Kani, said that "good progress" had been made but that the talks would "break [for] a few days."

Officials from France, Germany, the U.K and the U.S. have repeatedly warned over the last several weeks that the time to negotiate is running out. 

Iran has refused to engage in direct talks with the U.S. regarding the JCPOA and has suggested it will not resume treaty agreements until the U.S. revokes sanctions, a move the Biden administration has thus far rejected given Iran’s nuclear development. 

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"Getting that program back into a box through a return to mutual compliance with the JCPOA has proven more difficult through the course of this year than we would have liked to see," Sullivan said. "And we are paying the wages of the disastrous decision to leave the deal back in 2018.

"I'm not going to circle a date on the calendar next week or next month, but I will say that as they continue to move their program forward, it does imperil the fundamental viability of the JCPOA over time," he added.