A spokeswoman for Sen. John McCain confirmed the trip to Anadolu Agency.
“Senator McCain traveled to northern Syria this week to visit U.S. forces deployed there and to discuss the counter-ISIL campaign and ongoing operations to retake Raqqah," his office said.
A spokeswoman called the trip "a valuable opportunity to assess dynamic conditions on the ground".
The Wall Street Journal first reported the story, saying that in addition to meeting U.S. service members, McCain met with "Kurdish fighters", shorthand for the YPG, in Kobani.
Confirmation on the specifics mentioned in the story was not immediately available.
The YPG forms the backbone of the U.S.'s main partner in the counter-Daesh fight in Syria: the Syrian Democratic Forces (SDF).
But Washington's reliance on the YPG has strained relations with Ankara, who views the group as the Syrian branch of the PKK.
Both the U.S. and Turkey designate the PKK as a terrorist entity, but Washington has refrained from following suit on the YPG.
Supported by U.S. coalition air cover and on the ground personnel, the SDF is currently leading the charge to oust Daesh from its Syrian capital, Raqqah.
Meanwhile, President Donald Trump weighs new approaches to the anti-Daesh campaign. Defense officials are expect to present Trump with proposals by early next week.
It is unclear if the U.S. will continue to support the SDF, or will acquiesce to Turkish concerns regarding the YPG.
After meeting Turkish President Recep Tayyip Erdogan on Monday, McCain said the "change of administration in the United States presents an important opportunity to review and reassess current policy in Syria."
"The United States must work with Turkey to deal a rapid and lasting defeat to ISIL," he said, referring to the Islamic State of Iraq and the Levant, another name for Daesh.
McCain "looks forward" to working with Trump officials "to optimize our approach for accomplishing ISIL’s lasting defeat", his office said.