US President Donald Trump tried to justify the de facto green light he gifted his Turkish counterpart Recep Tayyip Erdogan for an assault seen as a blatant betrayal of Washington's erstwhile Kurdish allies.

But international condemnation of the Turkish assault was overwhelming and the response to the operation was discussed in an emergency meeting of the UN Security Council Thursday.

UN chief Antonio Guterres expressed his "deep concern" over the violence, while the five European members of the UN Security Council called on Turkey "to cease the unilateral military action."

Syrian Kurdish forces lost 11,000 personnel and played a key role in the years-long battle to eliminate the "caliphate" the Islamic State group had set up in the region.

In scenes all too familiar since the start of Syria's war more than eight years ago, civilians were seen abandoning their homes on Thursday, in vehicles or on foot with their belongings on their backs.

The Britain-based Syrian Observatory for Human Rights war monitor said more than 60,000 people had been displaced in less than a day.

"We're heading to the countryside because we're scared of renewed bombing and intensified clashes," said Rizan Mohammad, a 33-year-old who fled the Kurdish-majority city of Qamishli with his family after it was hit by Turkish artillery attacks.

After launching the assault with air strikes and intense artillery fire on Wednesday, the Turkish military and its Syrian proxies crossed the border into Kurdish-controlled areas.

- Renewed clashes -

On Thursday, Turkish jets carried out fresh strikes, the Observatory and a Kurdish military official told AFP.

Fighting broke out in several locations along the roughly 120 kilometre (75 mile) wide front where operations are focused, the sources said.

Turkish forces and allied rebels captured seven villages in the area, the Observatory said.

Fighting mostly centred around Tal Abyad -- one of the main Kurdish-controlled towns in the area coveted by Ankara.

Clashes also raged around Ras al-Ain further west, the other main town in the zone that Turkish media reports say is the first goal of the offensive.

According to the Observatory, which relies on a network of sources in Syria, at least 23 SDF fighters and nine civilians have been killed since the start of Turkey's assault.

On the other side of the border, five people including a baby were reportedly killed and dozens injured in Kurdish shelling on Turkish border towns.

AFP correspondents saw fighters crossing into Syria in dozens of vehicles.

Turkey, which has carried out two previous offensives into Syria since the start of the conflict, relies heavily on Syrian proxy fighters, many of who once battled President Bashar al-Assad's troops.

- Refugee warning to EU -

Erdogan wants a buffer zone to which he can send back some of the 3.6 million Syrian refugees his country hosts.

He warned the European Union on Thursday that the alternative if it criticised Turkey's military offensive was to allow the refugees to head to its shores instead.

"If you try to frame our operation there as an invasion... we will open the doors and send 3.6 million migrants to you," Erdogan said in a speech to the Turkish parliament.

Fourteen humanitarian organisations, including the Norwegian Refugee Council and Mercy Corps, warned that hundreds of thousands of people were in danger.

"An estimated 450,000 people live within five kilometres of the Syria-Turkey border and are at risk if all sides do not exercise maximum restraint and prioritise the protection of civilians," a joint statement said.

- How far in? -

The SDF, which have little armour and no air force, are unlikely to hold out very long against Turkish firepower in the flat and open terrain they are defending.

"The question that remains is how far can Turkey advance before international or regional actors, stop it," said Nick Heras, an analyst at the Center for a New American Security.

The assault appeared almost inevitable after Trump announced on Sunday that the US troops deployed in the area were pulling back from the border.

The withdrawal was implemented the next day, effectively clearing the way for Turkey to move in.

Trump has since tried to weather the international and domestic backlash, warning he would "wipe out" Turkey's economy if the operation was not conducted "in as humane a way as possible".

One of the Kurds' last hopes is that the prospect of IS prisoners breaking out and regrouping with increasingly active sleeper cells will spur the world into action.

The Kurds hold thousands of suspected IS fighters of dozens of nationalities whose countries of origin have refused to take back.

On Thursday, the Kurdish administration said a prison housing IS fighters in the region under its control was hit by Turkish bombardment, but did not specify the extent of the damage or if there were any jail breaks.

According to the Kurdish administration, some 12,000 men linked to IS are held in seven detention centres across northeast Syria.

Meanwhile, across the border in Iraq's Sulaymaniyah city, thousands of Kurds protested against Turkey's assault, chanting "Erdogan is a dictator" in solidarity with their Kurdish brethren.