The Russian envoy to the Iran nuclear talks said Wednesday that he had met with his US counterpart in Vienna, where the eighth round of talks is taking place.

Russia's Mikhail Ulyanov wrote on Twitter that he had met with the US special envoy to Iran, Robert Malley.

"Close consultations and coordination between the Russian and the U.S. delegations in the course of the Vienna talks constitute an important prerequisite for progress towards restoration of the JCPOA," he wrote.

Ulyanov also tweeted, that world powers which are patricipants in the JCPOA and the United States met Wednesday evening without Iran and "noted with satisfaction positive trens" in the talks.

The talks are about reviving the Joint Comprehensive Plan of Action (JCPOA) by bringing the United States back into the deal through sanctions lifting, and Iran through full implementation of its nuclear commitments.

The United States on Tuesday expressed caution over upbeat comments by Iran and Russia about talks in Vienna to salvage the 2015 Iran nuclear deal, saying it was too soon to say if Tehran had returned to the negotiations with a constructive approach.

With reporting by Reuters