Belgium's constitutional court on Thursday suspended a prisoner exchange treaty with Iran criticised for opening the way for a bomb-plot mastermind to return to Tehran.

Opponents of the Iranian government challenged the deal, which they argue was "tailor-made" to permit the release of Assadollah Assadi, an Iranian diplomat sentenced last year to 20 years in prison.

An Antwerp court convicted him of supplying explosives for a foiled 2018 bomb plot, targeting a meeting outside Paris of Iran's exiled opposition.

The Belgian government has said the treaty is the only way to secure the release aid worker Olivier Vandecasteele, who was detained in Iran in February.

The constitutional court ruled that the treaty was "suspended" pending a final ruling within the next three months.

"There is for the moment no longer any legal basis allowing this transfer," Francois Tulkens, a lawyer acting for the opposition group the National Council of Resistance of Iran, told AFP.

The Assadi case sparked a furious reaction in Iran.

The Antwerp court ruled that he had masterminded the bomb plot under diplomatic cover as an envoy to Austria -- and thus had no immunity in Belgium.

Tehran has demanded that Belgium recognise Assadi's diplomatic status and release him.

Aid worker Vandecasteele, 41, launched a hunger strike over the "inhuman" treatment by his captors in Iran, his family said last week, expressing worries about his failing health.

He is one of several Western nationals detained in Iran, in what foreign-based activists have said is a bid to extract concessions. Most of them also hold Iranian passports.