"The 8th round of the Vienna Talks... attended by China, France, Germany, Russia, United Kingdom, Iran and the United States, resume tomorrow in Vienna," EU spokesman Alain Matton tweeted.

Parties to the 2015 nuclear deal have been negotiating in the Austrian capital since last year to revive the landmark agreement with the US indirectly participating in the discussions.

After months of stalemate, progress has been made in recent weeks to revive the agreement that was supposed to prevent Iran from acquiring an atomic bomb, a goal it has always denied pursuing.

Former US president Donald Trump unilaterally withdrew from the pact in 2018 and reimposed tough economic sanctions on Iran, prompting the Islamic republic to begin pulling back from its commitments under the deal.

Talks on reviving the agreement were halted at the end of last month and the negotiators returned to their capitals for consultations.

Iran's top security official said Sunday that Washington and Tehran had so far failed to produce "balance" in their commitments during the Vienna talks.

"Despite limited progress in the #ViennaTalks, we are still far from achieving the necessary balance in the commitments of the parties," Supreme National Security Council secretary Ali Shamkhani said in a Twitter post.

"Political decisions in #Washington are requirements for balance of commitments to reach a good agreement."