Turkey kept up its assault on Kurdish-held border towns in northeastern Syria on Saturday on the fourth day of an offensive that is drawing growing international condemnation, even from Washington.
Buoyed by a night of steady advances, Turkish troops and their Syrian proxies pressed their advance into an area along the central section of the border that although Kurdish controlled, is mainly ethnic Arab.
The two towns at either end of the zone -- Ras al-Ain and Tal Abyad -- have come under heavy bombardment prompting civilian residents to flee in their tens of thousands.
"Turkish forces and allied rebel groups are attacking Ras al-Ain," the Syrian Observatory for Human Rights said.
Ankara's forces were advancing towards Ras al-Ain on three fronts under the cover of heavy artillery fire and air strikes, the Britain-based war monitor said.
An AFP correspondent near the town heard heavy exchanges as Turkey's mainly Arab Syrian allies headed to the front with rocket launchers.
Heavily outgunned fighters of the Kurdish-led Syrian Democratic Forces have taken heavy casualties resisting the Turkish assault.
At least 20 SDF fighters were killed in clashes overnight, the Observatory said, taking their losses so far to 74.
The heavy exchanges have led to civilian casualties on both sides of the border -- 20 so far on the Syrian side, according to the Observatory, and 17 on the Turkish side, according to Turkish reports.
An SDF official inside Ras al-Ain said his fighters had pushed back Ankara's forces but clashes were continuing.
In the countryside, Kurdish fighters have been losing ground.
Turkish forces overran 11 villages overnight, most of them near Tal Abyad, bringing to 23 the number they have taken so far, the Observatory said.
The SDF were the main ground partner in the protracted US-led campaign against the Islamic State group in Syria, losing 11,000 fighters before finally overrunning its self-proclaimed "caliphate" in March.
Facing with mounting condemnation, even within his own Republican Party, of his apparent abandonment of a loyal US ally, President Donald Trump has toughened his policy towards Ankara, threatening it with sanctions if its operation goes too far.
But Turkish counterpart Recep Tayyip Erdogan has voiced defiance and the Pentagon has reported no progress in its efforts to persuade Ankara to halt the offensive.