A military source from the Kurdish-led Syrian Democratic Forces (SDF) described the movement as a patrol running between the towns of Rmeilan to Qahtaniyah, 20 km (12 miles) to the west. The source said it would "not be a one-time" event.
The head of the SDF's media office could not immediately be reached for comment. The witness saw the vehicles outside the town of Qahtaniyah, roughly 6 km (4 miles) south of the border.
President Donald Trump announced this month that U.S. forces would withdraw from northeastern Syria, where the United States had allied with the SDF to oust Islamic State forces.
In response to a question about the reported troop movement, Colonel Myles Caggins, a spokesman for the U.S.-led coalition, said, "All Coalition military operations are de-conflicted with other forces operating in the region."
"We have begun repositioning Coalition troops to the Deir al-Zor region, in coordination with our SDF partners, to increase security (and) continue our mission to defeat (Islamic State) remnants," Caggins added.
The U.S. military said last week it was reinforcing its position in Syria with additional assets, including mechanized forces, to prevent oil fields from being taken over by remnants of the Islamic State militant group or others.
Trump said last week a small number of U.S. troops would remain in the area of Syria "where they have the oil." Syria's oil wells are principally located in Deir al-Zor province, well south of the Turkish-Syrian border.