"I do not believe Bashar al-Assad is sincere," Sarkozy told news TV channel i>tele in an interview. "Sadly I do not believe this ceasefire."
Sarkozy, waging an uphill battle for re-election in a vote that opens on April 22, said he had discussed the issue with U.S. President Barack Obama in a conference call on Thursday.
"I believe, and I discussed this yesterday afternoon with Barack Obama, that at the very least international observers must be deployed to establish what's going on," he said.
"I firmly believe the international community should live up to its responsibilities and create the conditions for (setting up) humanitarian corridors," he said.
Syrian opposition activists called mass protests for Friday to test a fragile, day-old ceasefire, and international pressure mounted for Damascus to comply fully with a UN-backed peace plan.
Sarkozy and Obama called on Syria on Thursday to adhere "scrupulously and without conditions" to a UN-backed plan to end the violence in the country.