Iranian officials criticised Russian “interference” in the final stages of talks aimed at reviving Tehran’s nuclear deal with major powers, Iran’s semi-official Tasnim news agency reported on Monday.

Talks to revive the 2015 pact have been mired in uncertainty following Russia’s demands for a U.S. guarantee that the sanctions it faces over the Ukraine conflict would not hurt its trade with Iran.

Moscow raised the potential stumbling block on Saturday, just as months of indirect talks between Tehran and Washington in Vienna appeared to be headed for agreement.

Foreign Minister Sergei Lavrov said Russia wanted a written U.S. guarantee that Russia’s trade, investment and military-technical cooperation with Iran would not be hindered in any way by the sanctions.

Tasnim cited Iranian officials on Monday saying the Russian demands were aimed at securing its interests in other areas and were “not constructive”.

By postponing the revival of the agreement and delaying Iran’s return to the oil market, Russia was seeking to raise crude prices and increase its own energy revenue, the news agency said, without citing a source for that assessment.

Iran’s top security official Ali Shamkhani said on Monday negotiators were evaluating new components that had affected the talks in Vienna. Shamkhani said on Twitter Iran was adapting initiatives to accelerate an agreement.

U.S. Secretary of State Antony Blinken sought to dispel talk of obstacles to the nuclear deal, saying on Sunday that the sanctions imposed on Russia over Ukraine had nothing to do with it.

The 2015 deal between Iran and the United States, France, Britain, Germany, Russia and China, eased sanctions on Tehran in return for limiting Iran’s enrichment of uranium, making it harder for Tehran to develop material for nuclear weapons.

The accord fell apart after President Donald Trump withdrew the United States in 2018.

Russia’s concerns about the impact of Western sanctions on its dealings with Iran follow a push by senior Iranian officials for deeper ties with Russia since the election of Iran’s hardline president Ebrahim Raisi last year.

Iran’s top authority, Supreme Leader Ayatollah Khamenei, has been calling for closer ties with Russia due to his deep mistrust of the United States.