The EU's top diplomat confirmed on Tuesday that Johan Floderus, a Swedish diplomat working for the bloc, has been held captive in Iran for more than 500 days.
Arriving at a meeting on development in the Spanish city of Cadiz, EU foreign policy chief Josep Borrell told reporters that Brussels was pushing “relentlessly” for the 33-year-old's release.
It was the first time that an EU official has publicly confirmed the name of the prisoner, who was part of the bloc's delegation to Afghanistan.
“I want to say something about, if you allow me, a specific case, the case of Mr Floderus. He's a Swedish citizen who worked for the European Union and has been detained illegally in Iran for the last 500 days,” Mr Borrell said.
“I want to stress that I personally, all my team at all levels – European institutions in close co-ordination with the Swedish authorities, which have the first responsibility of consular protection – and with his family, have been pushing the Iranian authorities to release him.
“Every time we had diplomatic meetings, at all levels, we have put the issue on the table. Relentlessly.
“We have been working for the freedom of Mr Floderus and we will continue doing that in close contact with the family, respecting their will, and for sure with the Swedish government,” he said.
“This is very much in our agenda, in our heart and we will not stop until Floderus will be free.”
The EU had not wanted to discuss Mr Floderus' arrest publicly in order to respect his family's wish for privacy, lead foreign affairs spokesman for the EU's external affairs Peter Stano told The National.
On Monday, his family confirmed a New York Times report on the detention and told local media that they were “deeply worried and in despair.”
“Johan was suddenly and without reason deprived of his freedom on a vacation trip and has been in an Iranian prison for over 500 days,” the family told Swedish daily Aftonbladet.
“He must be released and allowed to come home immediately.”
Mr Stano wrote in an email that: “We still believe that it is in the interest of our colleague and for the efforts of the Swedish authorities to avoid discussing the details of the handling of this case publicly.”
“You have seen the statement of Johan’s family. It was the right thing to respect their position,” he added.
“We have a duty of care and we have the interest of our colleague at heart. We have no further information to disclose in the interest of our colleague.”
Quoting unnamed sources, Aftonbladet reported that Iran arrested Mr Floderus in the hope of exchanging him with an Iranian citizen, Hamid Noury, detained in Sweden.
Noury received a life jail term for his role in the Iranian regime's 1988 mass executions of thousands of opponents.
Prisoner swaps between Iran and European countries have happened before.
In May, Belgian humanitarian worker Olivier Vandecasteele, 42, was released after 15 months in detention for alleged spying in a prisoner swap for Iranian diplomat Assadollah Assadi, sentenced to 20 years for plotting to bomb an opposition rally in Paris.
Several capitals have accused Iran of practising “hostage diplomacy” – arresting western citizens to obtain concessions such as the release of detained Iranians.