"In this case now, we have very limited options, and look, it's concerning that the secretary of State ... said that the future's up to the people in Syria on what happens with Assad," Rubio said on the radio show "AM Tampa Bay." "In essence, [Tillerson was] almost nodding to the idea that Assad was gonna get to stay in some capacity."
"I don't think it's a coincidence that a few days later we see this," Rubio added during the interview, which was first reported by CNN.
In New York on Thursday, the US ambassador to the United Nations, Nikki Haley, condemned Assad's history of human rights abuses against his own people.
But she said Washington would focus on working with powers such as Turkey and Russia to seek a political settlement, rather than focusing on Assad.
"You pick and choose your battles," Haley told reporters.
"And when we're looking at this, it's about changing up priorities and our priority is no longer to sit and focus on getting Assad out."