Iran has been holding a Belgian man for the past four months under “espionage” charges, Belgium’s justice minister said Tuesday, as his country weighed a controversial prisoner swap treaty with Tehran.

The man was seized in Iran on February 24 and has been in “illegal” detention since, the minister, Vincent Van Quickenborne, told Belgian MPs without identifying him.

Belgium last year imprisoned an Iranian diplomat for 20 years after his conviction under “terrorist” charges for plotting a bomb attack outside Paris in 2018.

While Quickenborne did not give the detained Belgian’s identity, Iran International, a Saudi-financed media outlet based in London, reported that a 41-year-old Belgian former aid worker is detained in Iran.

The outlet said the Belgian’s arrest appeared to be another instance of Iran “imprisoning foreigners as hostages to exchange them with certain Iranians jailed in Western countries.”

Among those Iran is holding is a Swedish academic who also holds Iranian citizenship, Ahmadreza Djalali, who taught at a Brussels university. Iran also applied “espionage” charges to Djalali and has sentenced him to death.

Quickenborne said officials from Belgium’s embassy in Tehran had twice visited the jailed Belgian to give all possible assistance, and that his family had earlier Tuesday made public his detention.

“I cannot say more, at the express request of the family,” the minister said.

Belgium’s parliament on Thursday is to vote on whether to ratify a bilateral treaty with Iran that would open the way for prisoners in each country to be repatriated.

Quickenborne on Tuesday said as he presented the proposed treaty to MPs for debate that “if the bill is not fully approved, the threat to our Belgian interests and certain Belgian citizens will increase.”

Some US lawmakers, however, are pressing Belgium to ditch the proposed treaty, which was signed in March.

One, Randy Weber, a Republican representative in Texas, tweeted he was “shocked to find out that the Belgian gov has cut a deal with the world’s leading state-sponsor of terrorism and plans to send Iranian terrorists back to Iran to plot more terroristic acts.”

The imprisoned Iranian diplomat, Assadollah Assadi, was convicted by a Belgian court in February 2021 of attempted “terrorist” murder and “participating in the activities of a terrorist group.”

He was found guilty of supplying explosives for a bomb attack on June 30, 2018 event outside Paris held by the dissident National Council of Resistance in Iran (NCRI) group.

Information supplied by several European intelligence services allowed Belgium to thwart the attack by intercepting the car carrying the bomb.

A two-year investigation into the plot determined that Assadi was an Iranian agent operating under diplomatic cover. Assadi was arrested in Germany, where his claim to diplomatic immunity was denied because he was attached to Iran’s embassy in Austria, and extradited to Belgium for trial. He opted not to appeal against his sentence. Tehran has protested his conviction.

Lawyers for the NCRI, whose core is made up of a militant organization known as the MEK, said the proposed Belgium-Iran treaty was designed to allow Assadi to go back to Iran.

The controversy in Belgium over the treaty comes as European powers are trying to bring Iran and the United States back into compliance with a 2015 nuclear deal. That pact was badly weakened when former president Donald Trump pulled America out in 2018.

Iran has since leapt ahead with its uranium enrichment to a level putting it close to the point where it could produce nuclear weapons.