"At least 600 people originating from Russia and Europe are fighting on the side of the Syria opposition," Putin told the Saint Petersburg Economic Forum.
"So why deliver arms to illegal armed groups in Syria, if we do not have a clear understanding of who they are comprised of? Where would these arms end up?" he asked.
The European Union agreed end May to lift its embargo against arming Syrian rebels while Washington said last week it would send military aid to the opposition, without giving details on the kind of weapons it would provide.
"Who will fill this void? Which terrorist organisations?" he asked, reiterating that it is the Syrian people who must decide Assad's fate.
Moscow is a staunch supporter of Assad's regime, and Putin has said that Russia would not rule out sending fresh arms to the Syrian regime.