Turkish troops have entered north-eastern Syria, launching a ground offensive against Kurdish fighters - just days after US troops pulled out of the area.
Turkish forces, supported by Syrian militants, have launched a ground offensive into Syrian territory as part of its operation against Kurdish forces.
The assault follows a massive artillery bombardment on Kurdish-controlled areas in northeastern Syria on Wednesday, ahead of the invasion made possible by the withdrawal of US troops.
A spokesman for one of the pro-Turkish Syrian militant groups told AFP the land phase of the operation began in Tal Abyad, an area under the control of the Syrian Kurdish People's Protection Units (YPG).
Turkey's defence ministry said troops had struck 181 Kurdish "terror group" targets so far.
Turkish media said special forces and armoured vehicles led the ground invasion and had entered at several points along the border.
"The Turkish Armed Forces and the Syrian National Army have launched the land operation into the east of the Euphrates river as part of the Operation Peace Spring," the defence ministry announced on Twitter.
Thousands of people fled the Syrian town of Ras al Ain towards Hasaka province, held by the Kurdish-led Syrian Democratic Forces.
The Turkish airstrikes killed at least five civilians and three SDF fighters and wounded dozens of civilians, the SDF said. Monitoring groups claim at least 15 civilians were killed during the lengthy bombardment.
Kurdish troops said they repelled a Turkish ground push on the northern border during the first hours of combat.
"Ground attack by Turkish forces has been repelled by SDF fighters" in the Tal Abyad region, said SDF spokesman Mustefa Bali on Twitter
President Recep Tayyip Erdogan announced the start of the attack on Twitter and soon after jets and artillery targeted Kurdish positions along the full width of the border, sending thousands of civilians fleeing their homes.
The move had seemed inevitable since US President Donald Trump on Sunday announced a military pullback from the border, but the attack triggered international condemnation and an emergency meeting of the UN Security Council set for Thursday.
The White House said Wednesday the Turkish assault was a "bad idea" after it had effectively gifted Mr Erdogan a green light and smashed the US alliance with the Kurdish forces, who spearheaded five years of ground battles against the Islamic State group in Syria.
An AFP correspondent reported Turkish artillery fire in the Ras al-Ain border area and explosions as warplanes flew overhead.
As plumes of smoke billowed into the sky, families could be seen filing out of town, some walking and others piling into vehicles with their belongings.
The Syrian Democratic Forces (SDF), made up mostly of the main Kurdish militia in the region, reported that at least two civilians were killed in airstrikes.
SDF fighters armed with rocket launchers were seen deploying in the area, as Kurdish authorities called up civilians to defend against the assault.
Kurdish sources reported that at least 16 positions were struck in the first hours of the operation, to which the SDF responded with some cross-border artillery fire.
While the Turkish military and its Syrian proxies - Arab-dominated former Syrian rebels - had not yet crossed into Syria, an AFP photographer saw forces massing near the border.
The SDF called on the international community to impose a no-fly zone to protect against "an imminent humanitarian crisis".
Mr Erdogan, who dubbed the attack "Operation Peace Spring", says the offensive is necessary to curb the power of the SDF due to its ties with Kurdish insurgents inside Turkey.
He also wants a "safe zone" on the Syrian side of the border where Turkey could send back some of the 3.6 million refugees it hosts from the eight-year civil war.
Trump insists the United States did not abandon its Kurdish allies, but he vowed to pull his troops out of Syria last year and analysts now predict that the days of the US presence in Syria are numbered.
In the face of the onslaught, Kurdish authorities announced a general mobilisation, urging all civilians to "head to the border with Turkey... to resist during this delicate historical moment".
Kurdish leaders said they would hold their erstwhile US ally and the whole international community responsible for any "humanitarian catastrophe".
In Ras al-Ain, Kurdish-led security forces set up checkpoints and stockpiled tyres to set alight to blur the vision of Turkish military pilots, an AFP correspondent reported.
Ras al-Ain was one of the places from which US troops withdrew on Monday.
"We will not leave this land," said Kawa Sleem, a 32-year-old Ras al-Ain resident.
"War has been chasing us for years, and everyday Erdogan threatens us with a new attack," he added.
It was expected that Ras al-Ain and Tal Abyad would be the focus of the first assaults.
Kurdish forces have dug trenches and tunnels in both areas, covering streets with metal canopies to block the cameras of Turkish drones.
But the flat and open terrain favours the vastly superior Turkish military over the Kurds, who have no air force and limited equipment.
The Kurdish-led SDF say they lost 11,000 personnel in years of operations against IS that climaxed in March with a battle against the final bastion of the jihadists' caliphate in Baghouz.
Trump, who is campaigning for a second mandate, has faced a barrage of criticism, including from close allies in Washington, for appearing to leave US allies to their fate.
Senior Republican Senator Lindsey Graham argued the US administration had "shamelessly abandoned" the Kurds and warned he would "lead (an) effort in Congress to make Erdogan pay a heavy price".
There has also been a chorus of international concern, including from France - the top US partner in the anti-IS coalition - and Russia, now even more firmly the main foreign player in Syria.
Since 2015, Russia has been the main military backer of the Syrian government, which has seized on the policy shift from Trump to try to persuade the Kurds to accept the restoration of central government control.
The Kurds have warned that a Turkish offensive would reverse the military gains achieved against IS and allow the jihadist group's surviving leaders to come out of hiding.
IS claimed an overnight suicide attack by two of its fighters in its former Syria stronghold of Raqa, the latest evidence that jihadist sleeper cells remained a threat.
Turkey has said it would not allow a resurgence of IS but called on European countries to repatriate jihadist prisoners being held in Kurdish detention centres.