The European Union’s coordinator for Iran nuclear talks was briefly detained with colleagues at Frankfurt Airport by German police on his way to Brussels, in breach of diplomatic rules, he said on Twitter on Friday.

He said that he was returning from an official trip to Tehran and also holds a Spanish diplomatic passport, Enrique Mora was held without explanation, and German police took his passport and phones. About 20 minutes later, he tweeted that he had been released.

Mora, who is from Spain and has long led the EU delegation at the talks in Vienna, said the incident happened Friday morning when he was trying to take a connecting flight from Tehran to Brussels.

He wrote on Twitter that he received "not a single explanation" from German authorities for why he was detained. "An EU official on an official mission holding a Spanish diplomatic passport. Took out my passport and my phones."

Mora said the EU Ambassador to the United Nations in Vienna and the head of the EU’s Iran task force were also detained.

"We were kept separatedly," he wrote. "Refusal to give any explanation for what seems a violation of the Vienna Convention."

Neither German police nor Germany’s Foreign Ministry responded immediately to requests for comment.

The convention, covering diplomatic relations, states that holders of diplomatic passports benefit from immunity and should not be hindered in their travels.

Mora was in Tehran this week to help revive Iran's 2015 nuclear deal with world powers.