He told US broadcaster PBS that strikes would increase instability and lead to the spread of terrorism in the region.
He said there was "no evidence" that government forces had used chemical weapons against their own people.
Meanwhile, US Secretary of State John Kerry has once again warned that taking no action against Mr Assad's regime is riskier than launching strikes.
When asked at a news conference whether there was anything Mr Assad could do to avoid military action, Mr Kerry replied that he could hand over his entire stockpile of chemical weapons within the next week.
However, US officials later clarified that Mr Kerry was making a "rhetorical argument" rather than a serious offer.
Mr Kerry has been lobbying hard for military action against Syria during talks with EU and Arab foreign ministers in Europe.
The US Congress is due to debate whether to authorise intervention in Syria. Watch BBC video » | Monday, September 09, 2013
