U.S. armoured vehicles were seen on Thursday near the Syria-Turkey border in a part of northeastern Syria where they had not been observed since early October when Washington announced the withdrawal of American forces, according to a witness and Reuters video footage.
A military source from the Kurdish-led Syrian Democratic Forces (SDF) described the movement as a patrol running between the towns of Rmeilan and Qahtaniyah, which is 20 km (12 miles) to the west. The source said it would "not be a one-time" event.
The witness saw the U.S. military vehicles outside the town of Qahtaniyah, roughly 6 km (4 miles) south of the border.
Later on Thursday, Reuters video recorded from Turkish territory showed a convoy of around 20 military vehicles flying American flags moving through the Syrian border city of Qamishli, which is about 30 km (19 miles) west of Qahtaniyah.
The head of the SDF's media office could not immediately be reached for comment. Turkey's defence and foreign ministries did not respond to a Reuters request for comment.
President Donald Trump announced on Oct. 6 that U.S. forces would withdraw from northeastern Syria, where the United States had allied with the SDF to oust Islamic State forces, paving the way for a Turkish offensive against Kurdish militia forces in the area.
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 border, though one is located in the country's northeastern tip nearer to where the U.S. forces were spotted.
Iran, Russia and Turkey - the three remaining actors in the region wielding influence over the conflict - have all criticised the U.S. decision to maintain a presence near oil fields.
The reported troop movement comes after the expiration of a Tuesday deadline set by Turkey and Russia for the Syrian Kurdish YPG militia, the main component of the SDF, to pull back 30 km (19 miles) away from its border. Turkey made a similar deal with the United States, pausing its assault during the withdrawal.
While both Washington and Moscow have said that YPG fighters left the border area within the given time, Turkey's President Tayyip Erdogan has said Kurdish fighters still remained, and that Ankara and Moscow would begin joint patrols along the border at a depth of 7 km (4 miles) on Friday.
Ankara views the YPG as a terrorist organisation because of its links to Kurdish militants in southeastern Turkey, and aims to establish a "safe zone" in northern Syria cleared of the group, where it says up to 2 million Syrian refugees can settle.
UN Secretary-General Antonio Guterres, in Istanbul for a mediation conference, said on Thursday he welcomed efforts to end fighting in northeastern Syria through dialogue, but urged restraint.
"I repeat my call for maximum restraint, de-escalation and the protection of civilians and civilian infrastructure in all fronts of Syria," Guterres said.
Syrian state media reported on Wednesday that the Syrian army and Turkish troops clashed near the border town of Ras al Ain. Erdogan has warned that Turkey could launch a new offensive if YPG fighters do not withdraw from the border.
Iran and Russia have supported Syrian President Bashar al-Assad in the 8-1/2 year civil war, while Turkey backs rebels who fought for years to overthrow him.
Turkey-backed rebels said on Tuesday they arrested an undisclosed number of Syrian army soldiers in Tel Hawa, in the Ras al Ain countryside near the Turkish border. Hours later, Turkey's defence ministry said 18 people, suspected of belonging to Syrian government forces, had been captured near Ras al Ain.
Turkish Defence Minister Hulusi Akar said on Thursday that Ankara is in talks with Moscow to hand over the 18 individuals.