Iran and the United States concluded 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.

ROME—Iran and the U.S. are aiming to set the parameters for a new deal on Tehran’s nuclear program while leaving important details to be negotiated later, people familiar with the talks said.Aiming for such a framework to guide progress toward a deal could raise concerns among some U.