Fierce fighting is taking place around the Syrian border town of Ras al-Ain as Turkish forces continue their offensive in the north-east against the Kurds.
Turkey says it has taken the key town, but the Kurdish-led Syrian Democratic Forces (SDF) dispute this.
Turkey has also denied targeting US forces after the Pentagon said troops further west had come under fire.
Nearly 50 civilians have reportedly been killed on both sides of the border and 100,000 displaced since Wednesday.
President Donald Trump’s decision to withdraw US troops from the area effectively triggered the Turkish incursion against the SDF – the main Western allies in the fight against the Islamic State (IS) group.
Turkey accuses the Kurds of being terrorists and says it wants to drive them away from a “safe zone” inside Syria where it plans to resettle more than three million Syrian refugees currently in Turkey.
One major concern for the international community is the fate of thousands of suspected IS prisoners, including many foreign nationals, being guarded by Kurdish-led forces in the region.
What’s the latest on the ground?
The Turkish military and allied Syrian rebels are engaged in heavy clashes with the SDF in Ras al-Ain. War planes have been circling, the town has been hit by days of artillery bombardment, and intense gunfire has been heard.
Meanwhile a spokesman for the Turkey-backed Syrian fighters told Reuters news agency they had seized a strategically important road between Tal Abyad and Ras al-Ain and 18 villages had been captured during the advance.
On Friday, the Pentagon said its base near the northern Syrian town of Kobane – which was not included in the US withdrawal and where Turkey knew US forces were present – had seen shell fire from Turkish positions. There were no injuries.
Turkey said it had responded to fire from the area and had ceased its bombardment after being informed by the US.
The SDF are facing numerous Turkish ground and air assaults along a stretch of the Turkey-Syria border about 75 miles (120km) long, correspondents say.
More than 20 SDF fighters were killed in clashes overnight, bringing the death toll to 74, the UK-based Syrian Observatory for Human Rights (SOHR) said. The SDF gave a lower figure of 45.
Separately, Kurdish media reports said Kurdish female politician Hevrin Khalaf had been killed, although it was unclear how she had died.
Turkey’s military said one of its soldiers had died and three others had been wounded.
Some 49 fighters with the pro-Turkish rebels (known as the Syrian National Army) have also been killed.
What is the situation for civilians?
Some 28 civilians have died in Kurdish-held territory since Wednesday, many of them in the Tal Abyad area, according to SOHR.
Dozens have also been seriously injured. One hundred thousand people have already been forced from their homes, according to the UN, but aid groups warn the number could rise to 450,000.
The fighting has also forced Tal Abyad’s only public hospital to close.
Turkish sources say 17 civilians have been killed in Kurdish shelling on Turkey’s side of the border since the operation began – among them a Syrian baby.
Why is the offensive happening?
Kurdish leaders, whose militia fought with the US to defeat IS, accuse the US of stabbing them in the back after Mr Trump effectively gave Turkey the go-ahead to move into north-eastern Syria.
Mr Trump now says he wants the US to negotiate a truce between Nato ally Turkey and the Kurds. Turkish President Recep Tayyip Erdogan, however, says the military operation will continue.
Mr Erdogan says he wants to create a “safe zone” in northern Syria free of Kurdish militias which could also be home to Syrian refugees.
The incursion has been condemned by many in the US and other Western countries and pressure is building in Washington to get Turkey to stop.
President Trump spoke briefly about the situation as he prepared to head to a rally on Friday, saying: “We don’t want them killing a lot of people… if we have to use sanctions we will.”
What are fears about IS based on?
IS appears to be trying to take advantage of the Turkish incursion.
On Saturday it declared a new campaign in Syria, which the group said was to avenge its members’ detention in Kurdish-run prisons.
Five IS members broke out of a prison in the city of Qamishli on Friday after Turkish shelling nearby, a Kurdish spokesman said. IS also said it was behind a car bombing there on the same day.
The SDF say they are holding more than 12,000 suspected IS members in seven prisons, and at least 4,000 of them are foreign nationals. The exact locations have not been revealed, but some are reportedly close to the Turkish border.
Two camps – Roj and Ain Issa – holding families of suspected IS members are inside the “safe zone”.
On Friday the Kurdish-led authorities said discussions were under way on how to move the Ain Issa camp, which had been hit by shelling.
Turkey says it will take responsibility for the IS prisoners it found during its offensive.