Turkey launched a broad assault on Kurdish-controlled areas in northeastern Syria on Wednesday, with intensive bombardment followed by a ground offensive made possible by the withdrawal of US troops.
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.
That was followed late in the evening by the beginning of a ground operation, the Turkish defence ministry said.
The British-based Syrian Observatory for Human Rights (SOHR) said 16 members of the Kurdish-led Syrian Democratic Forces (SDF) militia were killed in the early hours of the Turkish offensive.
Both the SOHR and SDF reported heavy clashes near the city of Tel Abad.
Turkey's defence ministry said on twitter that its forces had struck 181 Kurdish "terror group" targets so far.
The spokesman for one of the pro-Turkish Syrian militant groups told AFP the land phase of the operation began in Tal Abad, and Turkish media reported special forces and armoured vehicles had entered at several points along the border.
The assault 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 Arab League announced late Wednesday it was convening an emergency meeting in Cairo on October 12 to discuss the situation.
'Operation Peace Spring'
The US withdrawal smashed its alliance with the Kurdish forces who spearheaded five years of ground battles against the Islamic State group in Syria.
Trump warned Wednesday that if the Turkish operation is not conducted "in as humane a way as possible", he would "wipe out" the country's economy".
NATO chief Jens Stoltenberg urged Turkey to show "restraint" in its operation against Kurdish forces in Syria, warning that the fight against the Islamic State group should not be put at risk.
The SDF called on the international community to impose a no-fly zone to protect against "an imminent humanitarian crisis".
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.
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 Kaws Seem, 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 Abad 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 favour 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 and Britain -- the top US partners 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.
Regional heavyweight Saudi Arabia also condemned the offensive, arguing it would have "negative repercussions on the security and stability of the region", while the United Arab Emirates deplored Turkey's "flagrant interference in Arab affairs".
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.