The Belgian government called on Sunday for Belgians in Iran to leave the country, mired in a violent crackdown on nationwide protests, because of the risk of arbitrary detentions.

“All Belgian visitors, including (dual) nationals, are at high risk of arrest, arbitrary detention and unfair trial. This risk also applies to people who are simply visiting Iran for tourism,” the government said in a statement.

“In the event of arrest or detention, respect for fundamental rights and the safety of individuals are not guaranteed.”

Officials said on Wednesday that Iran had imposed a 28-year jail term on a Belgian aid worker, stirring an already bitter debate over a stalled prisoner exchange treaty.

Olivier Vandecasteele was arrested in February and is reportedly being held in Tehran’s notorious Evin prison, in conditions that Belgian Justice Minister Vincent Van Quickenborne described as “inhumane.”

Belgium insists he is innocent, effectively held as a hostage in Tehran’s efforts to force Belgium to release an Iranian agent convicted of terrorism.

News of Vandecasteele’s sentence has revived debate in Belgium over a prisoner exchange treaty with Iran.

Prime Minister Alexander De Croo’s government has described this in the past as the only option for a transfer.

The treaty was signed with Iran earlier this year and, while not tailored explicitly for Vandecasteele, Brussels confirmed that he would have been eligible for exchange.

But last week, Belgium’s constitutional court suspended the implementation of the treaty pending a final ruling on its legality within the next three months.

Opponents of the Iranian government have 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 for supplying explosives.

An Antwerp court convicted Assadi of supplying explosives to a couple from Belgium who were to travel to Paris to target a meeting of Iran’s exiled opposition.

In Spain on Sunday, relatives and friends of Spanish football fan Santiago Sanchez, arrested in Iran on his way to the World Cup, demanded his release during a rally outside Tehran’s embassy in Madrid.