Turkey's President Recep Tayyip Erdogan addresses a conference of parliament speakers, in Istanbul. AP
Battles raged on Friday as Turkish President Recep Tayyip Erdogan vowed to press an assault against Kurdish forces in Syria, dismissing "threats" from other countries while the US warned Ankara of fresh sanctions.
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 otherwise of "serious consequences".
But Erdogan vowed 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 see 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 Daesh group and its now-defunct "caliphate".
In Al Hol, a camp holding relatives of Daesh 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.
However Russia on Friday blocked a UN Security Council statement calling on Turkey to halt its military offensive, diplomats told the media.
China followed Russia in halting the US-drafted text in its tracks, a diplomat added, speaking on condition of anonymity.
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.
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.
Kurdish forces were putting up stiff resistance but experts predict they will not hold out very long without outside assistance.
Ras Al Ain, Tal Abyad and other border towns between them have been almost emptied 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 into a school being used as 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 mostly Arab, as an area in which to send back some of the 3.6 million Syrian refugees who live on Turkish soil.
Aid groups have warned of yet another humanitarian disaster in Syria's eight-year-old war if the offensive is not stopped.
France, a key partner in the US-led anti-Daesh 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 Daesh fighters could break free.
Kurdish officials said five Daesh prisoners managed to escape from a Qamishli facility housing mostly foreign militants after shelling struck nearby.
A car bomb claimed by Daesh also went off Friday in Qamishli, one of the main towns in the Kurdish region, killing at least six people, officials and the Observatory said.
France called for a meeting of the anti-Daesh coalition to discuss growing fears that the militant 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 has already plucked two of the most high-profile Daesh militants to have been captured alive and spirited them out of Syria.
The ministry said in a statement that the attack violated Ankara's agreements and cooperation with Russia, Syrian President Bashar Al Assad's most powerful ally. It said Russia had been told in advance about the convoy.
US forces in Syria started pulling back on Monday from Turkish border areas, opening the way for Ankara’s threatened military invasion and heightening fears of a extremist resurgence.
The attack north of Hama city adds to pressures on Russian-Turkish agreements that have contained the war in northwestern Syria, the last major foothold of the insurgency against President Bashar Al Assad.
Turkey accused the United States on Tuesday of taking only “cosmetic steps” toward the creation of a so-called “safe zone” in northeast Syria and renewed Ankara’s threat of unilateral military intervention to form a buffer area along its border.
Russia could conduct a “wide range” of aggressive acts against Ukraine, Nato chief Jens Stoltenberg said on Friday, but he emphasised the window was still open to a peaceful and negotiated resolution.
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