Negotiations in Vienna aimed at reviving the Iran nuclear deal were called off indefinitely Tuesday after a last-minute demand by Russia upended what diplomats had hoped was the final stretch of the talks.

The European Union’s foreign affairs chief Josep Borrell announced the decision to “pause” the talks in a tweet, citing “external factors” for the break. Negotiators for the seven countries involved have spent most of the past year huddled in Vienna trying to find ways to revive the 2015 nuclear deal after President Donald Trump’s decision to withdraw the United States from the agreement in 2018.

Diplomats said Borrell was referring to Russia’s demand, first raised last Saturday, for U.S. sanctions relief to be applied to its future commercial dealings with Iran as a condition for participating in a revived deal.

A final text for a new agreement is “essentially ready and on the table,” Borrell said, adding that he and his team would remain in contact with all the participants to overcome the remaining obstacles and finalize an agreement.

But the open-ended pause could also potentially signal a break from which there is no return, putting to rest any hope that restoring the deal will be possible.

“It’s certainly serious. If you lose momentum at this late stage the dynamics shift in ways that it could become impossible to resume the talks,” said Esfandyar Batmanghelidj of the European Council on Foreign Relations.

A small number of outstanding differences still to be settled between Iran and the United States may also have contributed to the deadlock, diplomats said. They include how far the United States will go in removing terrorism designations from organizations such as the Islamic Revolutionary Guard Corps, guarantees regarding the lifting of U.S. sanctions and the details of a prisoner exchange, which could bring freedom for U.S. and other Western detainees held in Iranian jails.

In Washington, State Department spokesman Ned Price cited the complex nature of the final stages of a negotiation for the pause, adding that there are “external factors that are also interceding” in the effort to revive the Joint Coordinated Plan of Action, or JCPOA, as the deal is known.

He said the United States is not prepared to offer Russia any concessions on sanctions for the sake of reviving the Iran deal, stressing that the new sanctions on Russia are “wholly and entirely unrelated to the JCPOA.”

Mikhail Ulyanov, Moscow’s envoy to the talks, told reporters that the break could not solely be blamed on Russia. “There are others that need to settle their issues among themselves,” he said.

Iran was restrained in its comments after the pause was announced Friday. Foreign Ministry spokesman Saeed Khatibzadeh said on Twitter that he hoped the break would create “momentum” toward resolving the remaining issues. “The successful conclusion of talks will be the main focus of all,” he said.

Meanwhile, however, the clock is ticking on the patience of Western allies to hold out for an agreement while Iran continues to accelerate its nuclear program. Iran has now advanced its enrichment and stockpiles of uranium to the extent that it could now only be weeks away from the threshold required to build a nuclear weapon, and U.S. officials have warned that they will not allow the negotiations to drag on indefinitely.

U.S. officials have in the past raised the possibility of implementing a “Plan B” in the event the talks fail, without specifying what the plan would entail. The options raise from imposing even tougher sanctions to military action, potentially compounding the global instability triggered by the Ukraine war by adding a second war in the Middle East.

“We’re not in that hellscape yet. We are just stuck in purgatory,” said Batmanghelidj.

The talks in Vienna had been focused on laying out a timetable to bring the United States and Iran back into compliance with the 2015 deal. Under a new agreement, the United States would be expected to lift the harsh new sanctions imposed after Trump withdrew, and Iran would be required to dial back advances subsequently made in its nuclear program.

A deal was so close that a podium for the final ceremonies had been erected in the Palais Coburg hotel where the talks were held. In recent weeks Iran had increasingly signaled its willingness to finalize the arrangement, diplomats say.

But the outbreak of the Ukraine war has shifted the geopolitical backdrop to the negotiations, and it is now possible that the fate of the Iran deal will effectively become hostage to the course of the war, diplomats say.

The deal would herald a return to world markets of Iranian oil, potentially tempering the price hikes caused by the Russian invasion of Ukraine and the U.S. sanctions on Russian oil. Russia has no interest in seeing the oil price come down and may also feel it can use the Iran deal as leverage in future negotiations over Ukraine, analysts say.

“Vladimir Putin understands that reviving the Iran nuclear deal means much more to Joe Biden than him. Putin does not feel threatened by Iran’s nuclear advancement and Tehran’s isolation has served Russian interest,” said Karim Sadjadpour, a senior fellow at the Carnegie Endowment for International Peace.

Iran had initially expressed irritation at Russia’s unexpected demand and continued to signal that a deal was near. But starting Wednesday, Iranian messaging switched, with officials turning their blame toward the United States.