Salman al-Audah warns that their presence among the rebels might serve the Assad regime's claim that the rebellion is the work of terrorist groups.
He told the Al-Jazeera satellite channel Monday that the Syrian rebels need money, weapons and prayers.
Al-Audah said a few Muslim fighters joining the Syrian rebels won't change the course of the conflict, but it could undermine European support. He said Europeans could associate foreign Muslim fighters with terrorists.
Al-Audah's comments were carried on his website.
There are no reliable figures for the number of foreign fighters in Syria. Estimates put the number in the hundreds.