Ankara stepped up its assault on Kurdish-held border towns in northeastern Syria on Saturday, defying mounting threats of international sanctions, even from Washington.
Buoyed by a night of steady advances in the countryside, Turkish troops and their Syrian allies entered the battleground town of Ras al-Ain, sources on both sides said.
The Turkish defence ministry hailed its forces' capture of the first Kurdish-held town of the offensive so far.
But Ras al-Ain's Kurdish defenders denied the town had fallen and an AFP correspondent near the town said Turkish troops and their Syrian allies had entered but had yet to capture it.
The Kurdish-led Syrian Democratic Forces, who were the main ground partner in the US-led campaign against the Islamic State group, have taken mounting losses against the vastly superior firepower of the Turkish army.
At least 20 SDF fighters were killed in clashes overnight, taking their losses since the Turkish offensive began on Wednesday to 74, the Syrian Observatory for Human Rights, a Britain-based war monitor, said.
Turkish air strikes on Kurdish-held towns and intense artillery exchanges caused mounting casualties on both sides of the border, with 28 dead on the Syrian side, according to the Observatory, and 17 dead in Turkey, according to Turkish reports.
The Turkish army has lost four dead, according to the defence ministry and the state-run Anadolu news agency.
The town of Ras al-Ain and that of Tal-Abyad further west have been primary goals of the Turkish offensive and have both come under heavy bombardment.
They lie at either end of a section of the border which although Kurdish-controlled has an ethnic Arab majority.
Ankara says its forces' mission is to establish a safe zone run by its mainly Arab Syrian allies in which some of the 3.6 million mostly Arab refugees from Syria can be rehoused.
But the Kurds say that the Turkish invasion, which has led to an exodus of civilian residents, Arab as well as Kurdish, amounts to an attempt to redraw the ethnic map of the region at their expense.
The offensive has so far displaced some 100,000 people, according to the United Nations.
Roads leading out of the area have been filled with fleeing civilians, some on foot, other in vehicles piled high with their belongings.
Few had any idea when if ever they would be able to return to their homes.
"We always get displaced no matter where we go," said Yusra al-Saleh, who fled violence along Syria's northern border.
"We are destroyed," the 38-year-old told AFP.
The Kurdish Red Crescent said it would no longer dispatch medical teams to Ras al-Ain because its ambulances are being hit by Turkish fire.
It said one of its medical points south of Ras al-Ain was hit by Turkish fire on Saturday, wounding an ambulance driver and damaging the vehicle.
Aid groups have warned of yet another humanitarian disaster in Syria's eight-year-old war if the offensive is not stopped.
"More people are leaving on a daily base and those numbers will go up," the World Food Program said Saturday.
Most of those fleeing were heading east towards the city of Hasakeh, which has not been targeted by Turkey.
"Turkey's aim is to prevent further fleeing Syrian civilians from entering Turkey rather than genuinely providing protection," Human Rights Watch said Friday.
The SDF lost 11,000 fighters in the protracted US-led campaign against the Islamic State group before finally overrunning the jihadists' self-proclaimed "caliphate" in March.
President Donald Trump has faced a firestorm of criticism, even from his own domestic supporters, for abandoning a loyal ally.
The Turkish offensive began after Trump ordered US troops to pull back from the border and he stands accused of giving it a green light.
He has since toughened his policy towards Ankara and on Friday threatened crippling sanctions if the operation goes too far.
But Turkish counterpart Recep Tayyip Erdogan has voiced defiance and the Pentagon has reported no progress in its belated efforts to persuade Ankara to halt the offensive.
France, a key partner in the US-led anti-IS coalition, has threatened sanctions against NATO member Turkey.
French leader Emmanuel Macron said the Turkish offensive must stop "as soon as possible" in a phone conversation with Trump on Friday, the presidency said.
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.
Kurdish officials said five IS prisoners managed to escape from a facility in the border city of Qamishli housing mostly foreign jihadists after shelling struck nearby.
A car bomb claimed by IS also went off Friday in Qamishli, one of the main cities in the Kurdish region, killing at least six people, officials and the Observatory said.
The Kurdish administration says some 12,000 men are held in seven detention centres across Kurdish-controlled areas.
The US says it has already plucked two of the most high-profile IS jihadists to have been captured and spirited them out of Syria.