Sen. John McCain, R-Ariz., made a surprise visit to Syria to meet with coalition forces fighting against ISIS, according to a new report.

McCain, who met with Arab rebels against Syrian dictator Bashar Assad in 2013, traveled this time to a part of northern Syria controlled by the Kurdish fighters who have proven most effective in fighting ISIS.

"[McCain] traveled to northern Syria last week to visit U.S. forces deployed there and to discuss the counter-ISIL campaign and ongoing operations to retake Raqqa," McCain spokeswoman Julie Tarallo told the Wall Street Journal.

McCain's trip took place as Defense Secretary Jim Mattis is finalizing a preliminary plan for a "comprehensive strategy" on how to defeat the beleaguered terrorist organization. A U.S.-led coalition in Iraq is fighting to liberate western Mosul, the largest city ever captured by ISIS, while "the constriction" of the terrorist capital of Raqqa is in an earlier phase.

But the defeat of ISIS has to be planned with one eye on how Russia and Iran, whose forces are deployed in Syria in support of Assad, could move to take more control in the region. The Obama administration developed a plan to give the Syrian Kurds heavy weaponry in the run-up to Raqqa, but that's controversial for two reasons. First, the Kurds are regarded as terrorists by Turkey, which is a NATO ally. Secondly, even U.S. leaders who support working with the Kurds are skeptical that "an Arab town" such as Raqqa acquiesce to Kurdish occupation.

"You'll have radical Sunni extremist groups kick the Kurds out," Sen. Lindsey Graham, R-S.C., told the Washington Examiner. "So, what I would recommend the administration think about doing is replacing the Kurds, not completely, but the Turks are willing to go in on the ground, they're will to help us liberate Raqqa."