Iran and six world powers have made significant progress in talks to revive their 2015 nuclear deal but important issues still need to be resolved, the Iranian Foreign Ministry spokesman said on Monday.

Iran and the powers have negotiated in Vienna since April to work out steps that Tehran and Washington must take on sanctions and nuclear activities to return to full compliance with the nuclear pact.

"Each round of talks in Vienna could have been the final round. We should not rush. We have made significant progress but key issues remain," Saeed Khatibzadeh told a televised weekly news conference. "There is no stalemate in the talks."

After former U.S. President Donald Trump ditched the deal three years ago and reimposed sanctions on Iran, Tehran has been rebuilding stockpiles of enriched uranium, enriching it to higher levels of fissile purity and installing advanced centrifuges to speed up production.