US forces on Sunday patrolled an area in northeastern Syria bordering Turkey after renewed tensions between Ankara and Syrian Kurds, a spokesman and an AFP reporter said.
Three armored vehicles carrying soldiers wearing the US flag on their uniform arrived in the Kurdish-held northeastern border town of Al-Darbasiyah, the correspondent said.
Turkey last week raised threats against Kurdish forces in northeastern Syria, shelling their positions and flagging a possible new offensive.
The Kurds spearhead the Syrian Democratic Forces alliance, backed by the US-led coalition, that has been fighting ISIS in Syria.
Coalition spokesman Sean Ryan said Sunday's patrol was the second in a week, after a first one by US forces on Friday.
"The US forces' assurance patrols enable us to maintain safety and security in the region," he said, but are not carried out "on a regular basis".
An SDF spokesman said the US patrols, in coordination with the SDF, were directly linked to recent tensions between the Kurds and Ankara.
"They are not routine patrols. They are directly linked to these threats. The objective is to call on Turkey to stop its aggression," Mustefa Bali said.
Sunday's patrols were headed towards Ras al-Ain, around 50 kilometers to the west of Al-Darbasiyah along the frontier, he said.
The US State Department has said it had been in touch with both the SDF and Turkey to push for de-escalation.
Turkey accuses Syria's Kurdish People's Protection Units (YPG) -- which form the backbone of the SDF -- of being "terrorists".
In what appeared to be an attempt by Washington to appease Turkey, US and Turkish troops on Thursday launched joint patrols on the outskirts of the northern city of Manbij.
Although the YPG claim to have pulled out of the city after the SDF seized it from ISIS in 2016, Ankara has recently complained that the group still has a presence there, repeatedly threatening military action.
On Wednesday, Turkish shelling of Kurdish positions in the Kobane sector of northern Syria killed four fighters, according to Turkey's state-run Anadolu news agency.
On Tuesday, two days after another round of shelling of Kurdish posts in northern Syria, Turkey's President Recep Tayyip Erdogan said his country had completed preparations for a new operation to "destroy" Kurdish fighters.
Since 2016, Turkey has carried out two operations against Kurdish forces in Syria, the last of which saw Ankara-backed Syrian opposition fighters take the border city of Afrin in March.
