WASHINGTON (Reuters) - The sanctions imposed on Russia over its Ukraine invasion have nothing to do with a potential nuclear deal with Iran, U.S. Secretary of State Antony Blinken said on Sunday, after Russia's demand a day before on written guarantees from Washington that its punitive measures won't harm cooperation on Iran.

       In an interview with CBS's "Face the Nation" show, Blinken said a potential nuclear deal with Iran was still close, but cautioned that "a couple of very challenging remaining issues" were still unresolved.

       "The sanctions that have been put in place ... on Russia have nothing to do with the Iran nuclear deal and the prospects of getting back into that agreement," Blinken said. "These things are totally different and just are not, in any way, linked together. So I think that's irrelevant.

       Russian Foreign Minister Sergei Lavrov said on Saturday that the Western sanctions imposed over the war in Ukraine had become a stumbling block for the Iran nuclear deal, warning that Russian national interests would have to be taken into account.

       The announcement by Russia, which could torpedo months of indirect talks between Tehran and Washington in Vienna, came shortly after Tehran said it had agreed to a roadmap with the U.N. nuclear watchdog to resolve outstanding issues that could help secure the nuclear pact.

       (Reporting by Humeyra Pamuk; Editing by Daniel Wallis and Mark Porter)