Tens of thousands of people have fled their homes in northern Syria, as Turkish forces step up their cross-border offensive on Kurdish-held areas.
Turkish troops have encircled the border towns of Ras al-Ain and Tal Abyad and aid agencies fear the exodus could reach hundreds of thousands.
International clamour has increased for Turkey to halt the attack.
Turkey has defended its bid to create a "safe zone" free of Kurdish militias which could also house Syrian refugees.
Turkey regards the Kurdish militias of the Syrian Democratic Forces (SDF) - which have controlled the cross-border areas - as "terrorists" who support an anti-Turkish insurgency.
The SDF have been key allies of the United States in the battle against the Islamic State (IS) group.
However, it was after President Donald Trump's decision to pull US troops out of the area that Turkey launched its assault, sparking SDF accusations they had been "stabbed in the back".
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.
How is the offensive affecting people?
The International Rescue Committee aid organisation said that 64,000 people had already reportedly fled their homes. The UK-based monitoring group, the Syrian Observatory for Human Rights, gave a similar figure.
The IRC's Misty Buswell said: "If the offensive continues it's possible a total of 300,000 people could be displaced to already overstretched camps and towns still recovering from the fight against IS."
Another group of 14 humanitarian organisations, including the Mercy Corps, warned the figure could be 450,000.
Ms Buswell said IRC teams remained on the ground, although other reports suggest some aid groups have pulled back across the Turkish border.
Sevinaz, a resident of Ras al-Ain, told the BBC on Thursday morning: "I am outside the town with my sick mother. My brother is inside. I have been informed that my cousin might have been martyred. There is no safe place for anybody.
"I'm concerned about it being the last time that I see my city."
Rizan Mohammad, who fled the Kurdish-majority city of Qamishli after Turkish air strikes, told AFP news agency: "We're heading to the countryside because we're scared of renewed bombing and intensified clashes."
Deaths on the frontier
The BBC's Martin Patience in Turkish border town of Akcakale
At the border you can see columns of smoke rising from the Syrian towns being shelled by the Turkish military. They look largely deserted but Kurdish fighters positioned there are firing back.
Several shells landed close to where many media organisations have set up, including the BBC on the outskirts of Akcakale. Inside the town, several mortar shells killed three people, including a nine-month-old Syrian baby.
But in Turkey there is widespread support for the operation. A tribal chief at the border hailed it as "a great day for Turkey".
He said it would mean Syrian refugees could go back home and that he supported everything President Recep Tayyip Erdogan does. Now that the offensive has started, Turkey's unlikely to back down.
What's the latest on the fighting?
It's an often confused picture, with differing versions.
Turkey's Anadolu news agency said late on Thursday that 228 Kurdish militants had been "neutralised" since the start of the operation.
The SDF had dismissed a figure of 109 given earlier in the day by President Erdogan as an exaggeration but they have not provided exact casualty figures.
The Syrian Observatory said at least 29 SDF fighters had died, along with at least 17 Turkish-backed Syrian rebels from the Free Syrian Army. It also said the offensive had captured more than 10 villages in the Ras al-Ain and Tal Abyad area, where the SDF appear to be under severe pressure.
An SDF Twitter account said an attack by Turkish forces east of the Jalab river had been repelled, with three military vehicles destroyed and 22 enemy combatants killed.
The Kurdish Red Crescent said there had been 11 confirmed civilian deaths so far and 28 serious injuries, mostly in Ras al-Ain and the border town of Qamishli.
At least five people, including a Syrian baby, were reportedly killed in Kurdish shelling of Turkish border towns.
Turkey wants to create a "safe zone" running for 480km along the Syrian side of the border but says it will not advance deeper than a planned 32km (20-mile) limit.
What has the reaction been?
The UN Security Council discussed the situation on Thursday at the request of its current five EU members - the UK, France, Germany, Belgium and Poland.
The five called on Turkey to halt its military offensive, saying "renewed armed hostilities in the north-east will further undermine the stability of the whole region, exacerbate civilian suffering and provoke further displacements".
UN Secretary General Antonio Guterres expressed his "deep concern" at the rising violence.
US ambassador to the UN Kelly Craft said: "Failure to play by the rules, to protect vulnerable populations, failure to guarantee that IS cannot exploit these actions to reconstitute, will have consequences."
That followed a tweet from Mr Trump in which he said he was "talking to both sides", adding: "I say hit Turkey very hard financially & with sanctions if they don't play by the rules! I am watching closely."
He sent out another tweet later saying there were three choices, "send in thousands of troops and win militarily, hit Turkey very hard financially and with sanctions, or mediate a deal between Turkey and the Kurds". He later told reporters: "I hope it's going to be the last one."
Turkey for its part said it would take responsibility for the IS prisoners it found during its offensive.
Mr Erdogan has strongly defended the incursion, threatening to send some of the 3.6 million Syrian refugees it hosts to Europe if the Turkish offensive is described as an occupation.
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