Iran and the US are expected to conclude talks in Vienna soon, despite Iranian chief negotiator Ali Bagheri Kani leaving for Tehran on Monday.
The two countries were edging towards the conclusion of an 11-month effort to revive the defunct 2015 nuclear deal, formally known as the Joint Comprehensive Plan of Action, when Russia demanded a sanctions exemption, tying the nuclear talks to the Ukraine invasion.
The EU's top envoy Enrique Mora, who is co-ordinating the talks, tweeted: “Just to clarify. There are no longer 'expert-level talks.' Nor 'formal meetings.' It is time, in the next few days, for political decisions to end the #ViennaTalks. The rest is noise.”
Mr Mora's Twitter post followed the departure of Mr Bagheri on Monday for consultations in Tehran and reports in Iranian media that experts would further pursue talks in Vienna.
In Washington, White House press secretary Jen Psaki told reporters the US and its allies are “getting closer” to a nuclear deal with Iran but “important components” still need to be decided.
At the weekend, US Secretary of State Antony Blinken said a deal was close, but gave a warning that "nothing is done until everything is done", a now-common refrain among diplomats working on the nuclear deal.
The stakes are high in Vienna and, if talks collapse, world leaders have warned the consequences could be dire. Former president Donald Trump's exit from the JCPOA in 2018 pushed Tehran to expand enrichment. Without a new agreement, Iran could quickly go past its current levels.
Although Iran has long said that it would not be pursuing nuclear weapons, any perception that its programmes are headed that way carries the risk of igniting conflict in the region.
Further enrichment by Iran is also likely to push the West to impose further sanctions on the country and, in turn, push already high oil prices even higher.
Negotiators last week said a deal would be ready within days, but now representatives from France, Britain, and Germany have temporarily left, urging Iran and the US, the main players in the deal, to get it to the finish line.
Despite last week's optimism over the deal coming together, Russian Foreign Minister Sergey Lavrov threw a damper on chances of a resolution, tying Iran talks with the war in Ukraine.
Mr Lavrov asked for a guarantee from the US that its trade, investment and military-technical co-operation with Iran would not be subject to the sanctions recently imposed on Russia for its invasion of Ukraine.
On Monday, Mr Lavrov insisted on Moscow's demand by telling Iranian Foreign Minister Hossein Amirabdollahian in a phone conversation that the revived nuclear deal should not allow for any discrimination between participants. Iran's foreign ministry spokesman, however, said that while he understood Russia's issue, the Iran talks should not be affected by other issues and sanctions.
France, one of the few countries to keep relations with Russia, warned Moscow not to resort to blackmail over efforts to revive the deal.
Diplomats have said that several issues still needed to be resolved to revive the nuclear pact, under which Tehran limited its nuclear programme in return for relief from US, EU and UN sanctions.
While Tehran said it would not allow “any foreign elements to undermine its national interests”, Iran's top security official, Ali Shamkhani, called on Washington on Monday to make political decisions.
“Priority of Iranian negotiators is to resolve remaining issues that are considered red lines for Iran. Reaching a firm agreement requires new initiatives from all parties.” Mr Shamkhani tweeted.
Russia's concerns about the impact of Western sanctions on its dealings with Iran follow a push by senior Iranian officials for deeper ties since hardliner President Ebrahim Raisi took office last year.
Iran's Supreme Leader Ayatollah Ali Khamenei has called for closer ties with Russia due to his deep mistrust of the US.
