Sen. Lindsey Graham, R-S.C., warned of "a second 9/11" amid reports the Trump administration is pulling thousands of U.S. troops out of Afghanistan.

Graham returned from a trip to Afghanistan this week and said in a series of tweets that withdrawing the U.S. military from the volatile Middle East could have dire consequences.

"I have just returned from Afghanistan and can say — without hesitation — ISIS-K remains a direct threat to our homeland and they would dramatically benefit from a reduced American troop presence," he said.

"Our Afghan partners are incapable of subduing the threat posed by ISIS-K alone. In addition, our intelligence capability and military might cannot be replaced by any other regional ally when it comes to subduing ISIS-K in Afghanistan or Pakistan.

"Like in Syria, troop withdrawal should be based on conditions on the ground.

"The conditions in Afghanistan — at the present moment — make American troop withdrawals a high risk strategy. If we continue on our present course we are setting in motion the loss of all our gains and paving the way toward a second 9/11."

Trump announced Wednesday he will pull troops out of Syria. Secretary of Defense James Mattis announced his resignation Thursday — around the same time it was reported the White House is considering withdrawing thousands of U.S. troops from Afghanistan.