Rubio says US can’t defeat ISIS until Syria's Assad is removed

"There is no such thing as 'Assad, yes', but 'ISIS, no,'" Rubio said.

"There is no such thing as 'Assad, yes,' but 'ISIS, no,'" Rubio said. "This focus that you can defeat ISIS as long as Assad is there is not true. They are two sides of the same coin."

The senator continued: “As long as Bashar al-Assad is in power in Syria, you will have a reason for people to be radicalized in Syria and that's what's going to happen."

He added that other extremist groups operating in Syria could later step in to replace ISIS.

"In all this about ISIS, there's an al-Qaeda group growing in strength, this al-Nusra coalition, that are prepared to step into the vacuum left behind by a defeated ISIS. You cannot have a stable Syria without jihadist elements on the ground as long as Bashar al-Assad is in power.”

Tillerson earlier on "This Week" told Stephanopoulos that the U.S. hopes for a political process in Syria that could bring stability and allow the country's citizens to determine if they want to keep Assad as president.

"The Syrian people will lawfully be able to decide the fate of Bashar al-Assad," Tillerson said.