Belgian lawmakers Tuesday debated a prisoner exchange treaty with Tehran that critics oppose, saying it will release an Iranian diplomat convicted of terrorism.

Justice Minister Vincent Van Quickenborne urged the foreign relations committee of Belgium's lower house to back an accord that intelligence services believed could avert an increased threat against Belgian interests.

"People's lives are at stake," the minister said, adding one threat that had materialised was the detention of a Belgian man in Iran since February.

Many lawmakers spoke about the possibility of an exchange to secure the release of the man, first identified by Iran International as a 41-year-old aid worker, and Swedish-Iranian academic Ahmadreza Djalali, who had taught in Belgium and has been sentenced to death in Iran.

Iran has called for the release of Assadollah Assadi, sentenced to 20 years in prison in Belgium in 2021 over a foiled 2018 bomb plot. His was the first trial of an Iranian official for suspected terrorism in Europe since Iran's 1979 revolution.

Many Iranian groups and activists as well as former senior US officials have urged the Belgian parliament not to approve the treaty, citing increased risk of fresh Iranian terrorism.

Several lawmakers expressed concerns that it could encourage Iran or others to seize Belgians.

"Would this open the door to a sort of hostage diplomacy?" said Greens member Wouter De Vriendt.

The committee was due to vote on the bill on Tuesday but adjourned the session until Wednesday after nearly four hours of debate. It is likely to be put before the full 150-member chamber on Thursday.