Battles raged in Syria as Turkish President Recep Tayyip Erdogan vowed to press on with a deadly assault against Kurdish forces, while the Pentagon said Friday US troops came under artillery fire from Turkish positions.
Erdogan's pledge to keep up the offensive, which the UN says has displaced more than 100,000 people since it began on Wednesday, came as the US Treasury warned President Donald Trump was planning to activate "very powerful" sanctions on Ankara.
Trump, whose order to pull back US troops from the border this week effectively triggered the intervention, has faced a firestorm of criticism for appearing to give a green light to the push.
Turkey is targeting the Kurdish-led Syrian Democratic Forces (SDF), a key US ally in the five-year battle to crush the Islamic State group.
A Pentagon spokesman confirmed an explosion within a few hundred meters of its post close to the northern Syrian border, in an area "known by the Turks to have US forces present".
"All US troops are accounted for with no injuries," Navy Captain Brook DeWalt said in a statement. US forces had not withdrawn from their position near the town of Kobani, he said.
"The US demands that Turkey avoid actions that could result in immediate defensive action," warned DeWalt.
Earlier Friday, US Defence Secretary Mark Esper "strongly encouraged" Turkey to halt its offensive as a prelude to such negotiations, warning 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."
Treasury Secretary Steven Mnuchin said Trump had authorized — but not yet activated — new sanctions to dissuade Turkey from further offensive military action.
"We can shut down the Turkish economy if we need to," he said.
Turkey's third such operation since the start of the war in Syria has been met with international condemnation over what many see as the blatant US betrayal of the Kurdish forces, who have lost 11,000 fighters in the campaign against IS.
As the offensive went into its third day, the SDF were fending for themselves, trying to repulse multiple ground attacks along a roughly 120-kilometre (75-mile) stretch of the border.
The Britain-based Syrian Observatory for Human Rights monitor said the Turkish forces and their Syrian proxies — mostly Sunni Arab ex-rebels — were launching air strikes, heavy artillery and rocket fire.
In Al-Hol, a camp holding relatives of IS suspects that 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 and China on Friday blocked a UN Security Council statement drafted by Washington calling on Turkey to halt its offensive, diplomats told AFP.
The Syrian Observatory for Human Rights 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 monitor, 54 SDF fighters have also been killed while Turkey has reported the deaths of four soldiers.
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.
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