Syrian President Bashar Assad said the U.S.-led airstrikes in Syria have not been effective in the fight against the Islamic State group. In fact, he said in an interview that aired Sunday on CBS' "60 Minutes," the terrorist group has been gaining strength and expanding its reach.

The group, also known as ISIS, seized a large swath of Iraq and Syria during the summer, a bloody rampage that has seen thousands slaughtered and featured the beheadings of captives. The barbaric actions prompted the United States and its allies to launch an air campaign aimed at dislodging the militants.

Assad said though the airstrikes may "have local benefit ... if you want to talk in terms of ISIS, actually ISIS has expanded since the beginning of the strikes."

Assad blamed the spread of ISIS and its ideology in part on Saudi Arabia, which he said is a "medieval system that's based on Wahhabi dark ideology," an ideology shared by ISIS. He also called Turkish President Recep Tayyip Erdoğan a "Muslim Brotherhood fanatic" and "megalomaniac," and accused Erdoğan of supporting terrorist immigration to Syria.

Assad said the majority of those who have been subjugated by ISIS support his government, and the rest of Syria is afraid of the militants.

"It's not traditional war. It's not about capturing land and gaining land. It's about winning the hearts and minds of the Syrians," Assad said.

Assad denied his soldiers are responsible for most of the civilian deaths across the country despite reports that allege that is the case. "That's impossible. I mean the logic has no leg to stand on," Assad said. Assad brushed aside charges his military used chlorine gas in recent weeks and dropped barrel bombs from helicopters, inviting an "impartial delegation" to investigate.

Assad said the only way he would leave office is if he sensed he did not have the support of the people. But, he said, while he'd be willing to negotiate with the United States, it's not Washington's business whether he remains in power.

"We have Syrian citizens, who can decide this. No one else," Assad said. "Whether they want to talk about it or not. This is not something we're going to discuss with anyone."