Foreign Minister Sergei Lavrov said during an annual press conference he was "optimistic" about the direction of talks.

"There's real progress. There's real desire -- primarily between Iran and the United States -- to understand specific concerns and how these concerns can be considered in the general package" of documents, Lavrov said.

The 2015 deal between Iran and world powers -- the United States, France, Britain, Russia, China and Germany -- offered Tehran relief from crippling sanctions in return for curbs on its nuclear programme.

But the unilateral US withdrawal from the agreement in 2018 under former president Donald Trump prompted Tehran to walk back on its commitments.

Talks to restore the accord began last year but stopped in June, when Iran elected ultraconservative President Ebrahim Raisi. They resumed in November.

"They are making good progress at the moment, touch wood. We think they will come to an agreement," Lavrov said.

After a rocky start, Iran said Monday that the talks had seen "relatively satisfactory progress".

However, this week France said the discussions were "too slow" and the United States warned that there were only "a few weeks left" to save the accord.