Annan, who resigned in August blaming "finger-pointing" at the United Nations Security Council for the impasse in his mediation, called for a political solution based on an agreement reached by world powers in Geneva in June.
"I don't see a military intervention in Syria. We left it too late. I'm not sure it would not do more harm," he told the Graduate Institute in Geneva on Tuesday night.
"Further militarization of the conflict, I'm not sure that is the way to help the Syrian people. They are waiting for the killing to stop. You find some people far away from Syria are the ones very keen for putting in weapons.
"My own view is that as late as it is we have to find a way of pouring water on the fire rather than the other way around."
France and Britain have urged the European Union to ease an EU arms embargo on Syria so that rebels can get more weapons.
Annan's successor, Lakhdar Brahimi, has pursued the Geneva plan, but has failed to bridge gaps between the United States and Russia, which resists Western demands for Assad's removal.
The agreement by foreign ministers of the Security Council's five permanent members called for a transitional government in Syria, but left open what part Assad might play.
"Once you talk of a transitional government with full executive authority, it means the existing government is on its way out and you are going to work to make a change. But they didn't do that. They left Geneva and started fighting again," Annan said of the divided world powers.
The United Nations says about 70,000 people have been killed in Syria, a figure Annan called "a gross under-estimation".
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