In a telephone press conference, US State Department spokesman Ned Price reported that there had been some progress in the last round of talks, but it was too early to know whether Tehran, in the current round, had returned to the table to build on those gains.

European negotiators in Vienna renewed their warning that the accelerated steps of Iran's nuclear programme were about to reach a point where the agreement signed in 2015 would be completed.

In a press summary, negotiators added that the time for the Vienna talks to revive the Agreement was weeks, not months.

Negotiators stressed the need to work intensively to discuss substantive contentious issues and to find practical solutions to them.

Iran and Russia had expressed optimism about the talks launched this week, despite the announcement by Western States that negotiations were proceeding too slowly.

The United States expressed caution over optimistic statements by Iran and Russia regarding the Vienna talks, saying it was still too early to say whether Tehran had returned to negotiations with a constructive approach.

Iranian Foreign Minister Hossein Amir Abdel-Lahian said that an agreement would be possible if the other parties demonstrated "good faith," while Russian Envoy Mikhail Ulyanov noted that a working group was making "undisputed progress" in the eighth round of talks.