Prisoner exchange frees Olivier Vandecasteele after 15 months in exchange for Assadollah Assadi, who handed off explosives for use at an Iranian opposition rally outside Paris

Neither Tehran, nor Brussels immediately confirm exchange; Oman state TV says both countries have transferred prisoners to Muscat ahead of release

Olivier Vandecasteele, 41, has been in custody since February; family says he is being held hostage as Tehran demands freedom of man jailed in Belgium for terrorism

Pact has been slammed by an Iranian dissident group, some US lawmakers and Amnesty International as paving way for release of Iranian diplomat jailed on terrorism conviction

Brussels says man has been in 'illegal' detention since February 24 arrest; prisoner swap with Tehran could see release of convicted terrorist

Tehran says 20-year sentence given to Assadollah Assadi over supplying explosives for thwarted attack is 'illegal and in flagrant violation of international law'