The EU's foreign policy chief says the trip by the coordinator of the talks to Tehran has unblocked the negotiations to revive the 2015 nuclear deal between Iran and world powers.

Josep Borrell made the remarks at a G7 meeting in northern Germany on Friday as Enrique Mora arrived back in Europe, saying he believes that two months of deadlock in diplomatic efforts has ended.

He described Iran's response as being "positive enough" after Mora had delivered a message that things could not continue as they were, describing his trip as “better than expected.”

He did not say what "unblocked" means and if the suspended talks in Vienna will resume.

"These things can not be resolved overnight. Let's say the negotiations were blocked and they have been unblocked and that means there is the prospect of reaching a final agreement."

Enrique Mora arrived in Tehran for a two-day trip on Tuesday and met with Ali Bagheri-Kani and other Iranian officials, but with no news about the results of the meeting until late evening in Iran.

Talks to restore the deal with world powers have stalled since March, chiefly over Tehran's insistence that Washington remove the US designation of Foreign Terrorist Organization of Iran’s Revolutionary Guard (IRGC), which is the only example of a sovereign state’s armed forces to be included.

A member of Iran’s nuclear negotiating team said on Thursday that removing the terrorist designation of the IRGC is not enough and Tehran wants other guarantees.