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.

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

"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.

There was no immediate comment from Frankfurt police.

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."

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.