Turkey remained defiant against mounting international pressure to curb its military offensive against Kurdish militants in Syria, raising tensions with Washington as Vice President Mike Pence headed for Ankara on Wednesday to demand a ceasefire.

Battles raged in the key Syrian border town of Ras al-Ain as dawn broke on Wednesday, with Kurdish fighters trying to hold off the onslaught by Turkish-backed forces, now in its second week.

The fighting has triggered a flurry of diplomacy among major powers, with US President Donald Trump dispatching Pence along with his top diplomat Mike Pompeo to Turkey amid the greatest crisis in relations for decades between the NATO allies.

The Kremlin said it would host President Recep Tayyip Erdogan for a meeting with his Russian counterpart Vladimir Putin in the coming days, to ensure the operation does not turn into an all-out war between Turkey and Syria.

Russia has stepped into the void caused by Trump's withdrawal of US troops from northern Syria, deploying patrols to prevent clashes between Syrian and Turkish forces.

Trump — facing mounting criticism in Washington over his decision to pull 1,000 troops out of the conflict zone, as well as an unrelated impeachment inquiry — has hit back at Erdogan, slapping sanctions on three cabinet officials and raising tariffs on Turkish steel.

Pence said he would meet with Erdogan on Thursday and “voice the United States' commitment to reach an immediate ceasefire and the conditions for a negotiated settlement”, his office said in a statement.

He reiterated that Trump will pursue “punishing economic sanctions” until a resolution is reached.

But Erdogan remained unfazed by the pressure, telling reporters: “They tell us 'to declare a ceasefire'. We can never declare a ceasefire.”

The operation has widespread support in Turkey, where decades of bloody insurgency by Kurdish militants has killed tens of thousands of people.

But Western powers are spooked that the operation is endangering the battle against the militant Islamic State (IS) group, which was led on the ground by Kurdish forces.

Thousands of IS prisoners are held in Kurdish-run camps in the region.

French Foreign Minister Jean-Yves Le Drian said on Wednesday that the camps were not “currently” under threat from the operation, but Europe has taken an increasingly tough line with Turkey.

Britain and Spain became the latest powers to suspend military exports to Turkey on Tuesday. Canada made a similar move.

Kurds try to defend border

The Kurdish-led Syrian Democratic Forces (SDF) have mounted a desperate defence to the east of Ras al-Ain, using tunnels, berms and trenches.

An AFP correspondent said clashes around the town were ongoing on Wednesday despite Ankara's repeated claims it had captured the area.

The Syrian Observatory for Human Rights said Kurdish fighters had launched “a large counterattack against Turkish forces and their Syrian proxies near Ras al-Ain” on Tuesday and reported “fierce combat” in the west of the town as well as in Tal Abyad.

Since launching their assault on October 9, Turkish-backed forces have secured more than 100 kilometres (60 miles) of border, but Ras al-Ain — Siri Kani in Kurdish — has held out.

Erdogan, who like Trump faces political difficulties at home, wants to create a buffer zone stretching 30 kilometres (20 miles) from the border into Syrian territory.

He wants to destroy Kurdish hopes of an autonomous enclave that could serve as a launching pad for attacks in Turkey, as well as resettle some of the 3.6 million Syrian refugees it is hosting.

“God willing, we will quickly secure the region stretching from Manbij to our border with Iraq,” Erdogan said.

The offensive has killed dozens of civilians, mostly on the Kurdish side, and displaced at least 160,000 people.

Russia fills void

Syrian forces have returned to the region for the first time in years, raising their flag in Manbij as part of a deal with the Kurds.

Russia said its military police were patrolling a zone separating regime and Turkish troops, in cooperation with Ankara.

With Trump's critics saying that he handed over US allies and stretches of Syria to Russia, the United States tried to play down Moscow's role.

“The number of Russians is very, very limited. But it only takes a few Russians with a big Russian flag to get everybody to pay attention,” a senior administration official told reporters in Washington.

Meanwhile, at least three French women escapees were “retrieved” by IS, according to messages they sent to their lawyer seen by AFP.

An SDF official on Twitter Tuesday said more IS relatives had tried and failed to escape the overcrowded camp of Al-Hol in eastern Syria.