An EU spokesman played down the incident, which he said was "solved quickly" but which Mora had declared "seems a violation of the Vienna Convention" on diplomatic protection.

Germany's federal police confirmed that officers had "stopped and checked three European diplomats" based on information relating to their flight on the Tehran-Frankfurt route.

"After it turned out that there was no relevance to the information, the diplomats were able to continue their journey after about 40 minutes," the statement said.

Earlier, Mora had tweeted: "Retained by the German police at the Frankfurt airport on my way to Brussels, back from Teheran. Not a single explanation.

"An EU official on an official mission holding a Spanish diplomatic passport. Took out my passport and my phones," he said.

A short time later he added: "Now released along with my two colleagues, the EU Ambassador to UN Vienna and the head of the EEAS Iran task force."

The EEAS is the European External Action Service, the EU equivalent of a foreign ministry, with its own diplomats.

"We were kept separated. Refusal to give any explanation for what seems a violation of the Vienna Convention," Mora complained.

- 'Issue over' -

The Vienna Convention on Diplomatic Relations grants legal privileges to diplomats and diplomatic missions, and is one of the treaties that underpins all international relations.

Asked about the incident, EU foreign policy chief Josep Borrell said "the issue is over, he took the plane and is travelling in accordance with his mission."

German interior ministry spokesman Maximilian Kall said: "The checks that took place were solely about the travel route, about indications that existed about this flight, but were in no way connected to these three people.

"That's why the three diplomats could of course continue their journey after brief questioning. It had nothing do with these three people."

Mora had been in Tehran on Wednesday for talks with Iran's chief negotiator on efforts to salvage the international deal limiting Iran's nuclear programme in exchange for sanctions relief.

Mora has played a key role as an intermediary between the United States and Iran during a year of on-off talks in Vienna to restore the pact, formally known as the Joint Comprehensive Plan of Action (JCPOA).

Borrell said the latest visit had "gone better than expected" and expressed optimism about negotiations.