President Donald Trump, whose order to pull back US troops from the border this week effectively triggered the intervention, said Washington would now seek to broker a truce.

US Defence Secretary Mark Esper "strongly encouraged" Turkey to halt its offensive as a prelude to such negotiations, warning of "serious consequences" if it did not hault the assault.

But Erdogan swiftly responded by vowing that the assault "will not stop".

"Now there are threats coming from left and right, telling us to stop this," he said. "We will not step back."

His pledge to carry on with the push, which the UN says has displaced more than 100,000, came as US Treasury Secretary Steven Mnuchin said Trump was planning to activate "very powerful" sanctions on Ankara.

"We hope we don't have to use them but we can shut down the Turkish economy if we need to," he said.

The third such Turkish operation since the start of the war in Syria has been met with fierce international condemnation over what many saw as the blatant betrayal of a faithful ally.

The Kurdish forces targeted by Turkey were the US-led coalition's main ground partner in years of battle against the Islamic State group and its now-defunct "caliphate".

In Al-Hol, a camp holding relatives of IS suspects which lies outside the area targeted by Turkey, women started riots Friday that Kurdish forces swiftly put down.

The risk that thousands of the jihadists they still hold could break free on the back of the Turkish assault could yet spur the international community into action.

But as the offensive went into its third day, the Kurdish-led Syrian Democratic Forces were fending for themselves, trying to repulse multiple ground attacks along a roughly 120 kilometre (75 mile) long segment of the border.

"There is heavy fighting between the SDF and the Turks on different fronts, mostly from Tal Abyad to Ras al-Ain," the Syrian Observatory for Human Rights said.

The Britain-based war monitor said the Turkish forces and their Syrian proxies -- mostly Sunni Arab ex-rebels -- were launching air strikes, heavy artillery and rocket fire.

- Exodus -

The monitor said four civilians were killed in Tal Abyad when an air strike hit the car in which they were fleeing the fighting, while another three were shot dead by snipers around the border town.

That brings the civilian death toll to 17 on the Syrian side, while 17 have also been killed in Turkey.

According to the Observatory, 54 SDF fighters have also been killed while Turkey has reported the deaths of four soldiers.

Outgunned Kurdish forces were putting up stiff resistance but experts predict they will not hold out very long without outside assistance.

The Observatory and a Kurdish military source said several Arab families in the border area had sided with Turkey, raising sleeper cells to attack from behind SDF lines.

Ras al-Ain, Tal Abyad and other border towns between them have been almost emptied of their population in a huge wave of displacement.

Most of those fleeing were heading east towards the city of Hasakeh, which has not been targeted by Turkey.

"What does Erdogan want from us?" asked one woman, as she and her family settled in a school turned by local authorities into an emergency shelter.

"Is it all simply because we are Kurds?"

Erdogan wants to create a buffer zone between the border and territory controlled by Syrian Kurdish forces, who have links with Turkey's own Kurdish rebels.

He also plans to use the strip, which he envisions will be about 30 kilometres deep and is mostly Arab, as an area in which to send back some of the 3.6 million Syrian refugees who live on Turkish soil.

- Ceasefire? -

Aid groups have warned of yet another humanitarian disaster in Syria's eight-year-old war if the offensive was not stopped.

France, a key partner in the US-led anti-IS coalition, has threatened sanctions against NATO member Turkey.

Turkey is still far from having reached the goals of its military invasion but the risk appears to be growing that detained IS fighters could break free.

Russian President Vladimir Putin said he doubted Turkey would be able to ensure IS prisoners stay behind bars.

"I'm not sure whether the Turkish army will be able to take this under control -- and how soon," he said. "This is a real threat to us."

Kurdish officials said five IS prisoners managed to escape from a Qamishli facility housing mostly foreign jihadists after shelling struck nearby.

A car bomb claimed by IS also went off Friday in Qamishli, one of the main towns in the Kurdish region, killing at least six people -- civilians and members of the security forces -- officials and the Observatory said.

France called for a meeting of the anti-IS coalition to discuss growing fears that the jihadist organisation could regroup if Turkey's invasion creates a security vacuum.

The Kurdish administration says some 12,000 men are held in seven detention centres across Kurdish-controlled areas.

The US already plucked two of the most high-profile IS jihadists to have been captured alive and spirited them out of Syria.