The French and Colombian governments say they have clear indications the leftist Revolutionary Armed Forces of Colombia holds 35-year-old Romeo Langlois.
The FARC has not issued any official statements on Langlois' disappearance.
Three soldiers and a police officer were killed when FARC guerrillas attacked troops accompanied by Langlois on a cocaine lab-destruction mission. Colombia's defense minister says survivors saw Langlois suffer a bullet wound in his arm and flee toward the rebels.
Top EU foreign affairs official Catherine Ashton called for the "immediate and unconditional liberation" of Langlois in a statement Wednesday.
The Committee to Protect Journalists and other journalist groups have made similar appeals, stressing that under international law journalists are non-combatants and should be treated as such by all parties to a conflict.
Some journalists' groups have noted the FARC's vow in February to abandon ransom kidnapping. In early April, the movement released 10 soldiers and police officers who it called its last remaining "political prisoners."