FIRST ON FOX: A new nuclear agreement with Iran is expected within days, a European diplomat told Fox News on Friday – coming after weeks of renewed talks in Vienna to revive the 2015 Iran nuclear deal.

The diplomat cautioned, however, that any deal involving Iran, China, Russia, and the U.S. could always unravel. Meanwhile, on Thursday, Iran’s chief negotiator told reporters that a deal would be done within 24-48 hours.

IRAN NEARING NUCLEAR THRESHOLD, WITH US OPTIONS TO STOP IT NARROWING, REPORT WARNS 

The 2015 Joint Comprehensive Plan of Action (JCPOA) set limits on Iran’s nuclear activity in exchange for a significant rollback of international sanctions. The Trump administration pulled the U.S. out of the deal amid concerns that the agreement did not do enough to curb Iran’s nuclear ambitions -- including the sunset clauses on restrictions.

Iran eventually pulled out of the deal after the Trump administration re-imposed sanctions as part of its maximum pressure campaign, and Tehran has been ramping up its enrichment since then. The International Atomic Energy Agency (IAEA) said this week it believes Iran has significantly increased its stockpile of highly enriched uranium, but that it is unable to verify the exact size of the stockpile due to limits imposed on inspectors by the regime.

The Biden administration has sought to re-enter the deal and talks began in Vienna last year to revive the deal. Those talks initially stalled amid demands for significant sanctions relief by Iran’s hard line regime. 

As for the question of Russian diplomats negotiating in Vienna during a European flight embargo on Russia over its invasion of Ukraine, the diplomat told Fox that there can always be exceptions for diplomats.

With Russia’s involvement in the deal, it raises the possibility of financial benefit for Moscow in a new deal, like payments to take Iranian uranium. The diplomat told Fox that the West will find a way to enable the deal, and says the concerns are "nothing that can’t be overcome."

BIDEN ADMINISTRATION WAIVES SANCTIONS ON IRANIAN CIVILIAN NUCLEAR ACTIVITIES AS DEAL TALKS HANG IN BALANCE

The State Department did not comment when asked whether the Biden administration has concerns about needing Russia’s cooperation or having to carve out sanctions to enable to agreement. 

Last month, Secretary of State Antony Blinken signed a waiver that restored the ability of foreign companies and other countries to participate in civilian nuclear and safety projects in Iran without facing U.S. sanction.

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A senior State Department official told Fox News Digital at that time that the waivers were due to growing non-proliferation concerns with respect to increasing stockpiles of enriched uranium by Tehran, and that without the waiver, "detailed technical discussions with third parties regarding disposition of stockpiles and other activities of nonproliferation value cannot take place."

Fox News’ Gillian Turner and The Associated Press contributed to this report.