LAUSANNE, Switzerland — In an unusually upbeat assessment, Iran’s top nuclear official said Tuesday his government’s main disagreements with the U.S. and its negotiating partners have been resolved and expressed optimism about meeting a late March deadline for a framework deal.

Now in their second extension, the talks have made headway in recent weeks. The sides have moved closer on limitations on Iran’s nuclear activities that could be retooled to make weapons. In exchange, the West would progressively lift economic and political sanctions.

Still, the comments by Iranian nuclear chief Ali Akhbar Salehi were among the most promising to date. “The main issues have been closed,” he told Iranian state TV. “I hope that in the remaining time we can close this.”

In separate comments to reporters in Lausanne, he said common ground on one “final item” still was missing. If that is resolved, “we can say that on technical issues, things are clear on both sides.

“Of course there are many details, but I can say that, as a whole, I am optimistic” about a deal before deadline, he said.

The sides are working to meet two target dates — a framework in the next two weeks that lays down the outlines of a final deal by the end of June.

A senior U.S. official was less bullish, saying the sides had made progress but still had a ways to go in eliminating differences on what Tehran had to do for a gradual end to sanctions.

U.S. Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif have taken the lead in what formally remain talks between Iran on one side and the United States, Russia, China, Britain, France and Germany on the other.