The jihadist group ISIS "may be one of the biggest we have ever faced," Sen. John McCain said Sunday, as a result of a failure by the U.S. to intervene early in Syria's violent civil war.

"One of the biggest mistakes ever made, in recent times, was the president’s overruling his entire national security team, including the secretary of state [Hillary Clinton], that argued two years ago for providing weapons for the Free Syrian Army," McCain said Sunday on CBS's "Face The Nation." "That was a seminal moment."

This week, President Obama said the U.S. has "no strategy" yet to take on the growing threat posed by ISIS, which is encroaching farther into Iraq from Syria and could threaten the Iraqi capital city, Baghdad.

Obama's remark has been met with criticism in recent days by Republicans and Democrats alike, including McCain.

"We have to have a clear strategy because ... to defeat ISIS — not contain, not stop, but defeat ISIS," McCain said Sunday, "because they are a direct threat, over time, to the United States of America."