The United States military patrolled in a Kurdish-controlled area next to the Turkish border on Sunday in what appeared to be a show of strength after fresh tensions between Ankara and Washington.

The patrol consisted of three armoured vehicles carrying soldiers wearing the U.S. flag, France 24 said citing a news wire report. The vehicles were seen in the Kurdish-held border town of Al-Darbasiyah, it said.

Political tensions between the United States and Turkey boiled over into crisis this year after Turkey invaded an enclave in the Kurdish-controlled north of Syria to battle militants there and refused to release a U.S. pastor from detention, prompting President Donald Trump to impose economic sanctions on the NATO ally in August.

Last week, Turkey shelled the positions of Kurdish forces in Syria’s north and President Recep Tayyip Erdogan threatened a new offensive. Turkey says the militants, a lynchpin of the U.S. battle against Islamic State (ISIS), are inseparable from the Kurdistan Workers’ Party (PKK), recognised as terrorists by the United States and European Union.

Coalition spokesman Sean Ryan confirmed the patrol had occurred following another by U.S. forces on Friday. He said they were not carried out on a regular basis, according to the report.

A spokesman for the Syrian Democratic Forces (SDF), which the Kurds dominate, said the troop movements were a direct response to Turkey’s actions.

"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. The military unit then headed off along the border, he said.

The U.S. State Department has said it had been in touch with both the SDF and Turkey to push for de-escalation, according to the report.

U.S. and Turkish troops began joint patrols around the outskirts of the northern city of Manbij last week in a step designed to ease tensions. Turkey says Kurdish forces there have not left the city, despite U.S. assurances that they would do so.

A Turkish court released U.S. pastor Andrew Brunson from two years of custody early last month after finding him guilty of terrorism charges but freeing him for time served. Washington and Ankara simultaneously ended tit-for-tat sanctions at the weekend, in place since early August.