"The US is delivering weapons, ammunition and food to the Islamic State forces. It is an operation to use ISIL mercenaries as a destabilizing force [in Syria]," Bennett said.
He warned that Pandora's Box has already been opened and that these mercenary forces had turned into a "toxic residue" in Syria's political landscape.
He also slammed Washington's Middle East policy and touted Russia's role in defeating Islamic State militants, a task that he said the White House has failed to implement.
"I really hope that Vladimir Putin' Russian influence in attacking ISIL will help create a coalition of unified countries with the honest desire and ambition to defeat the fanatics," Bennett said.
Additionally, he urged the United States to stop funding mercenaries, which he said Washington "amazingly fails to do."
Last year, the Obama administration announced a program on training Syrian rebel fighters to combat the Islamic State in Iraq and Syria.
Earlier this week, General Lloyd Austin, head of the US military's Central Command, told the Senate Armed Services Committee that Washington's training program is behind schedule, and that the military's target of training 15,000 rebels in three years will not be met on time. It has only trained several so far.