Iran and the United States are set to begin a fifth round of high-stakes nuclear talks in Rome on Friday amid growing skepticism in Tehran about the chances of a deal as Washington hardens its position.

Iranian and U.S. negotiators resumed talks on Friday in Rome to resolve a decades-long dispute over Iran's nuclear ambitions, Iranian media reported, despite Tehran warning that clinching a new deal might be insurmountable amid mutually exclusive demands.

Iranian officials say an American demand that Tehran stop enriching uranium could push them to walk away from negotiations.