Nicolas Henin was held with four hostages - Americans James Foley and Steven Sotloff, and Britons David Haines and Alan Henning - who were all murdered by British extremist Mohammed Emwazi.
He was released in April 2014. In a short video by the opposition group the Syria Campaign, he warned that IS strikes were a "trap" that would push locals into the hands of extremists.
"We are just fuelling our enemies and fuelling the misery and disaster for the local people …" he said.
"The winner of this war will not be the party that has the newest, the most expensive or the most sophisticated weaponry but the party that manages to have the people on its side."
He blamed the rise of IS extremists on Syrian president Bashar al Assad's repression of the revolution and the lack of action by the global community, which led to "Syrians living in total despair".
"The international community failed to assist the Syrian democrat as they were yelling for their freedom," he said in the message coinciding with Britain's parliamentary vote in favour of airstrikes.
The key to forcing the "collapse" of IS was to find a political solution engaging the local people and imposing no-fly zones on all regions held by the Syrian opposition, he said.
Describing the jihadis who held him captive, he said: "They have a vision of the world that is self-coherent.
"This vision is like a parallel world.
"This Western jihadi was always telling me about the moving matrix, that now we are living in a different matrix.
"They follow the news intensively and every single event that happens in the world they will see it as a confirmation of their belief."
But he believes last summer's refugee crisis was a blow to IS propaganda and that extremists hated seeing hundreds of thousands fleeing a "Muslim land" to go to what IS regard as the "land of the unbelievers".
