Sen. Lindsey Graham, R-S.C., warned in an interview with Fox News Saturday that withdrawing U.S. troops from Afghanistan would lead to “another 9/11," saying that America can’t “outsource” U.S. national security to the Taliban.

“If we left tomorrow, the Taliban cannot be trusted to take care of ISIS and Al Qaeda,” Graham told chief intelligence correspondent Catherine Herridge. “They don't have the capability. So when you ask our intelligence community…what would happen to America if we withdrew all of our forces. They tell us without any hesitation it would lead to another 9/11 in a matter of time.”

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When asked what the timeframe for such a situation would be, Graham said it would be “years not decades.”

The U.S. invaded Afghanistan in the wake of the 9/11 attacks in 2001, beginning an 18-year involvement – the longest in U.S. history. The conflict has killed more than 2,000 American troops, injured more than 20,000 and cost nearly a trillion dollars since 2001.

President Trump announced a 4,000-troop increase in August 2017 as part of an effort to break the stalemate in the country. But he has been moving toward agreeing to a phased withdrawal of troops. The Washington Post reports that U.S. Special Envoy Zalmay Khalilzad is negotiating a deal to draw down troops from 14,000 to 8,600.

The two sides have been discussing an agreement in which the U.S. withdraws from Afghanistan, in return for the Taliban promising to ensure that Afghanistan does not become a haven for terrorists and Al Qaeda.

Trump met last week with his top security officials about the way forward in Afghanistan. "Just completed a very good meeting on Afghanistan," the president tweeted afterward.

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But Graham says that the Taliban don’t have the capacity to remove the terror group, and said he had been briefed by General Austin “Scott” Miller that the Afghan version of ISIS, ISIS-K, is also trying to set up operations against America and are perhaps more lethal than Al Qaeda.

“It would be insane to outsource our national security to the Taliban when it comes to ISIS,” he said.

Graham says that the U.S. could reduce safely to 8,600 but anything below that is a “high-risk strategy.” He plans to introduce legislation to create a backstop measure to require the secretaries of state and defense to certify that dropping troop levels would not endanger U.S. national security.

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He told Fox News that a combination of the intelligence community plus a strong counterterrorism force is a “strong insurance policy” against an ISIS attack.

“You take those two things off the table it's just a matter of time 'til we get hit,” he warned.