Talks between Iran and the United States over reviving the 2015 nuclear deal appear to have stalled after Tehran suggested there were new obstacles and Washington said hard issues remained. The Biden administration would likely lift some of the punishing sanctions on Iran if the deal was brought back. That task has been made more urgent by Vladimir Putin’s invasion of Ukraine and the U.S. decision to retaliate by banning all Russian oil imports, which potentially removes from Western markets more than 10 million barrels per day of oil.

Some of that lost oil supply could be made up by Iran, which pumped an average 2.4 million barrels per day in 2021 but has been able to sell less than half of what it produces due to sanctions.

Opponents of Iran warn that even as Ukraine scrambles geopolitical calculations and the U.S. turns its attention to Russia, the Biden administration shouldn’t take pressure off the Islamic Republic. The U.S. considers Iran a state sponsor of terror along with its Islamic Revolutionary Guard Corps, an elite military unit that plays a key role in the oil industry and supporter of Hezbollah and other militant groups active throughout the Middle East.

Diplomats in Vienna had been preparing for a meeting to bring Iran back into compliance with the 2015 deal’s restrictions on its rapidly advancing nuclear activities and the United States back into the accord it left in 2018 by re-imposing sanctions on Tehran. But Russian Foreign Minister Sergei Lavrov unexpectedly demanded sweeping guarantees that Russian trade with Iran would not be affected by sanctions imposed on Moscow over its invasion of Ukraine. Western powers said the demand was unacceptable and Washington has insisted it will not entertain.

Russia’s demand initially angered Tehran and appeared to help it and Washington move towards agreement on the few remaining thorny issues, diplomats said, but a sudden volley of public comments by Iranian officials including Supreme Leader Ali Khamenei suggested the winds had shifted.

“US approach to Iran’s principled demands, coupled with its unreasonable offers and unjustified pressure to hastily reach an agreement, show that US isn’t interested in a strong deal that would satisfy both parties,” Khamenei’s top security official Ali Shamkhani said in English on Twitter. “Absent US political decision, the talks get knottier by the hour.”