This picture taken on October 15 shows smoke rising from the Syrian town of Ras al-Ain, from the Turkish side of the border at Ceylanpinar district in Sanliurfa, on the first week of Turkey's military operation against Syrian Kurdish fighters. Photo: AFP

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

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 cease-fire 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 cease-fire.' We can never declare a cease-fire."

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 Islamic State group, which was led on the ground by Kurdish forces. Thousands of IS prisoners are held in Kurdish-run camps in the region.