WEISSENHAUS, Germany (REUTERS) - The European Union's foreign policy chief said on Friday (May 13) that he believed there had been enough progress during consultations between his envoy and Iranian officials in Teheran this week to relaunch nuclear negotiations after two months of deadlock.

Talks to revive Iran's 2015 nuclear deal with world powers have been on hold since March, chiefly over Teheran's insistence that Washington remove the Islamic Revolutionary Guard Corps from the United States' list of designated terrorist organisations.

Speaking as talks coordinator Enrique Mora arrived back in Europe, Mr Josep Borrell said Iran's response had been "positive enough" after Mr Mora had delivered a message that things could not continue as they were.

"These things can not be resolved overnight," Mr Borrell told reporters at a Group of Seven foreign ministers meeting in northern Germany.

"Let's say the negotiations were blocked and they have been de-blocked and that means there is the prospective of reaching a final agreement."

The broad outline of the deal that aims to revive the accord, which restrains Iran's nuclear programme in return for relief from economic sanctions, was essentially agreed in March.

However, it has since been thrown into disarray after last-minute Russian demands and the dispute over the US Foreign Terrorist Organisation list.

Western officials are largely losing hope that it can be resurrected, sources familiar with the matter have said, forcing them to weigh how to limit Iran's atomic programme even as Russia's invasion of Ukraine has divided the big powers.

"It has gone better than expected - the negotiations were stalled, and now they have been reopened," Mr Borrell said.

Iranian Foreign Minister Hossein Amirabdollahian said Mr Mora's trip had been "an opportunity to focus on initiatives to resolve the remaining issues".

"A good and reliable agreement is within reach if the United States makes a political decision and adheres to its commitments," he added.

A French diplomatic source said on Thursday that he saw little chance of the US agreeing to remove Iran's elite security force from its list of foreign terrorist organisations any time soon.

Mr Mora has been in Teheran this week in what has been described as the last chance to salvage the 2015 accord, which then US President Donald Trump withdrew from in 2018.

Britain, China, France, Germany and Russia are also parties to the accord.

In a bizarre incident, Mr Mora and his team were held at Frankfurt airport for several hours on return from the Iranian capital on Friday.

"We were kept separated. Refusal to give any explanation for what seems a violation of the Vienna Convention," he said on Twitter.

The German authorities did not immediately comment.

Iran's official IRNA news agency alleged, without evidence, that Israel was behind the incident.

"What has happened in Frankfurt has to do with opposition to the progress in the nuclear talks... The Zionist lobby has influence in the German security apparatus," it said.