Turkish and Allied forces faced tough Kurdish resistance on Friday as they struggled to seize key Syrian border cities, on the third day of a broad offensive that caused a civil exodus.
President Donald Trump, whose order to withdraw US troops from the border this week effectively triggered the invasion, said Washington would now seek to negotiate a truce.
The third Turkish operation of this type since the beginning of the war in Syria encountered a fierce international condemnation, even among Trump's own allies, for what many saw as the flagrant betrayal of a faithful ally.
Kurdish forces attacked by Turkey were the main land partner of the US-led coalition in years of battle against the militant group of the Islamic State (IS) and its now defunct "caliphate."
The risk that thousands of the jihadists who still possess will be released after the Turkish assault could push the international community into action.
But two days after the offensive, the Syrian Democratic Forces led by the Kurds defended themselves, trying to repel multiple ground attacks along a border segment approximately 120 kilometers long.
"There are strong clashes between the SDFs and the Turks on different fronts, mainly from Tal Abyad to Ras al-Ain," said the Syrian Human Rights Observatory.
The British-based war monitor said Turkish forces and their Syrian representatives, mostly former Sunni Arab rebels, were deploying airstrikes, heavy artillery and rocket fire.
"The SDFs are using tunnels, trenches and berms" in their defense operations, the Observatory said.
The monitor said four civilians were killed in Tal Abyad when an airstrike hit the car in which they were fleeing the fighting, while three others were shot dead by snipers in the border city.
That brings the number of civilian deaths to 17 on the Syrian side, while seven have also been killed in Turkey.
According to the Observatory, 41 SDF fighters were also killed, while Turkey reported the death of a single soldier.
Kurdish counterattacks during the night led to the recovery of two of the 11 villages they had lost since the start of the Turkey-led assault on Wednesday.
The Observatory and a Kurdish military source said several Arab families in the border area had sided with Turkey, raising sleeping cells to attack from behind the SDF lines.
A AFP Correspondent in the Ras al-Ain area said new units of former Syrian rebels were brought to break Kurdish resistance.
Ras al-Ain, Tal Abyad and other border cities between them have almost emptied their population in a great wave of displacement.
Most of the 70,000 people that the United Nations confirmed had been displaced traveled eastward to the city of Hasakeh, which has not been targeted by Turkey.
"What does Erdogan want from us?" A woman asked, referring to Turkish President Recep Tayyip Erdogan, when she and her family settled in a school that local authorities had turned into an emergency shelter.
"Is it all simply because we are Kurds?"
Erdogan wants to create a buffer between the border and the territory controlled by the Syrian Kurdish forces, which have links with the Kurdish rebels in Turkey.
He also plans to use the strip, which he plans to have about 30 kilometers deep and is mainly Arab, as an area in which to send back some of the 3.6 million Syrian refugees living on Turkish soil.
The area would be under Turkish control and administered by Syrian representatives, a movement that would hinder the return of displaced Kurds and permanently reshape the ethnic map of the area.
Stop the fire?
On Twitter on Thursday, Trump said he hoped to "mediate an agreement between Turkey and the Kurds," saying the alternatives were to send "thousands of troops" or hit Turkey with sanctions.
An American official explained that Trump had asked diplomats to try to negotiate a ceasefire and argued that sanctions against Turkey were not justified at this stage.
Doctors Without Borders (MSF) said in a statement that it had to close the hospital that supported Tal Abyad.
The bombing there led most people to leave, including hospital medical staff, he said.
Aid groups have warned of another humanitarian disaster in Syria's eight-year war if the offensive does not stop.
France, which was the main partner of the United States in the anti-IS coalition, has threatened sanctions against Turkey, a member of NATO.
Turkey is still far from having achieved the objectives of its military invasion, but the risk seems to be growing so that detained IS fighters can break free.
Russian President Vladimir Putin said he doubted that Turkey could guarantee that the prisoners of the Islamic State remain behind bars.
"I'm not sure if the Turkish army can take this under control, and how soon," he said. "This is a real threat to us."
France called a meeting of the anti-IS coalition to discuss the growing fears that the militant organization could regroup if the invasion of Turkey creates a security vacuum.
According to the Kurdish administration, some 12,000 men are detained in seven detention centers in areas controlled by the Kurds.
The United States has already ripped out two of the most prominent IS jihadists who were captured alive and removed them from Syria.