"I do not believe in Bashar al-Assad's sincerity, nor unfortunately in the ceasefire. I think we must deploy observers so that at the very least we know what is happening," he said in an interview with French television I-Tele.
"I am convinced that the international community must assume its responsibilities and create humanitarian corridors so those unfortunates who are being massacred by a dictator can escape," said Sarkozy, who is campaigning for re-election in an April-May presidential vote.
Sarkozy and US President Barack Obama had on Thursday demanded that Syria respect the terms of a peace plan drawn up by UN-Arab League envoy Kofi Annan.
The French presidency announced the joint call after the two leaders held videophone talks on a series of world crises, and warned that Assad's regime would be "judged on its actions".
"The two presidents urged the Syrian regime to scrupulously and unconditionally respect its undertakings in regard to the plan by the United Nations special envoy and the Arab League," the statement said.