The United States will beef up its military presence to protect oil fields in north-eastern Syria as Kurdish forces abandoned several positions.

Key points: Russian forces are now patrolling the Syrian-Turkish frontier

Russian forces are now patrolling the Syrian-Turkish frontier US Defense Secretary Mark Esper has hit out at Turkey for putting "us all [in a] terrible situation"

US Defense Secretary Mark Esper has hit out at Turkey for putting "us all [in a] terrible situation" The United States has no designs of giving up oil fields, saying the aim is to stop IS from having access to them

They did so to comply with a deal allowing Syria, Turkey and Russia to carve up their now-defunct autonomous region.

"The US is committed to reinforcing our position, in coordination with our SDF (Syrian Democratic Forces) partners, in north-east Syria with additional military assets to prevent those oil fields from falling back to into the hands of ISIS or other destabilising actors," a Pentagon official said in a statement.

The official did not provide any numbers or confirm reports that US armoured assets would stay by the oil fields, once used to fund the Islamic State group's short-lived "caliphate".

Russians launch military patrols

The announcement came as Russian forces began patrolling the Syrian-Turkish frontier, filling part of the vacuum left by a US troop withdrawal that effectively returned a third of Syria to the Moscow-backed regime of President Bashar al-Assad.

A patrol flying Russian flags was seen setting off from Qamishli, westwards along the border.

The Russian defence ministry said the patrol covered "more than 60 kilometres" between Qamishli and Amuda.

The Syrian Observatory for Human Rights reported that the Kurdish-led SDF had pulled out of some areas at the eastern end of the border on Thursday.

But fighters from the Kurdish People's Protection Units (YPG) — the main component of the SDF — remained in many positions along the 440-kilometre border, according to Observatory head Rami Abdel Rahman.

The Britain-based war monitor also reported clashes near the town of Tal Tamr between SDF fighters and some of the Syrian former rebels paid by Turkey to fight ground battles.

On Tuesday, Russia and Turkey signed a deal in Sochi that promised a ceasefire, while requiring Kurdish forces to withdraw to a line 30 kilometres from the border.

Turkish President Recep Tayyip Erdogan, who is embattled on the domestic political front, hopes to use the pocket to resettle at least half of the 3.6 million Syrian refugees his country hosts.

Under the Sochi deal, the area will remain under the full control of Turkey, unlike the rest of the projected buffer zone that will eventually be jointly patrolled by Turkey and Russia.

Turkey slammed at NATO meeting

As Kurdish troops withdrew, SDF commander Mazloum Abdi accused the Turkish-led forces of violating the truce on the eastern front of Ras al-Ain.

"The guarantors of the ceasefire must carry out their responsibilities to rein in the Turks," he tweeted.

NATO defence ministers also slammed Turkey for its military operation in Syria at the start of a two-day meeting in Brussels.

United States Secretary of Defence Mark Esper said Turkey was "heading in the wrong direction".

German Defence Minister Annegret Kramp-Karrenbauer said she and her French and British counterparts believed the Turkish-Russian "safe zone" agreement "does not provide a permanent basis for a political solution".

US Secretary of Defense Mark Esper, speaking at a think tank conference in Brussels before the NATO meeting, was more blunt, saying NATO member Turkey was "heading in the wrong direction".

"Turkey put us all in a very terrible situation and I think the incursion's unwarranted," Mr Esper said.

US to protect oil

Washington's insistence on maintaining a military presence in the oil fields in the country's far north-east corner, after abandoning other positions along the Turkish frontier, drew doubts and criticism.

But the Pentagon official, who insisted on anonymity, stressed it was to prevent a potentially resurgent Islamic State jihadist movement from retaking control of the fields.

"One of the most significant gains by the US and our partners in the fight against IS was gaining control of oil fields in Eastern Syria," the official said.

"We must deny IS this revenue stream to ensure there's no resurgence."

AFP