QAMISHLI, Syria — Syria's Kurds battled to hold off a Turkish invasion on Thursday after air strikes and shelling launched a long-threatened operation that could reshape the country and trigger a humanitarian crisis.

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.

Syrian Kurdish forces lost 11,000 personnel and played a key role in the years-long battle to eliminate Daesh (the so-called IS) in the region.

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

The broad offensive — which Erdogan dubbed "Operation Peace Spring" — drew international outrage and warnings, including from within Trump's own camp, and will be discussed in an emergency meeting of the UN Security Council later on Thursday.

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

Turkey was yet to unleash its full military might however, with the Syrian Democratic Forces — the autonomous Kurds' de facto army — holding off two incursion attempts.

The first one was conducted late Wednesday against Tal Abyad, one of the main Kurdish-controlled towns in the area where Ankara wants to set up a buffer zone stretching some 30 km (20 miles) into Syria.

The other push was attempted on Thursday around Ras Al-Ain further west, the other main town in the zone Turkish media reports say is the first goal of the offensive.

Our fighters "confronted a field incursion attempt by the Turkish occupation army on the axis of Tal Halaf and Aluk," the Kurdish-led Syrian Democratic Forces (SDF) said.

Turkish jets carried out fresh strikes in late morning, the Syrian Observatory for Human Rights and a Kurdish military official said.

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

According to the Britain-based Observatory, at least 19 SDF fighters and eight civilians were killed in the first hours of the assault.

Correspondents on the Turkish side of the border said they saw fighters crossing into Syria in dozens of vehicles.

Turkey, which has carried out two previous cross-border offensives into Syria since the start of the conflict, relies heavily on Syrian proxies for ground operations.

The Syrian fighters — most of them grouped under the banner of an outfit that calls itself the Syrian National Army — are mostly Sunni Arab former rebels who were defeated by the Damascus regime.

Erdogan, who is politically embattled at home, wants a buffer zone in which to 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 criticized 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.

In addition to feared deportations from Turkey, the latest flare-up was sending thousands of fearful civilians — both Kurdish and Arab — onto the roads across the targeted Kurdish-controlled areas.

"A military offensive could displace 300,000 people and sever life-saving humanitarian services," the International Rescue Committee warned.

The SDF — which have little armor and no air force — are unlikely to hold out very long against Turkish firepower in the flat and open terrain along the border.

"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 had seemed 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".

Senior Republican senator Lindsey Graham argued the US administration had "shamelessly abandoned" the Kurds.

There has also been a chorus of international concern, including from France and Britain — the top US partners in the anti-Daesh coalition — and Russia, now even more firmly the main foreign player in Syria.

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

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

In the confusion of the Turkish operation's early hours, US forces took two high-profile militants into custody and moved them out of the country, Trump said.

The pair are both Britons — part of a group dubbed "The Beatles" which stands accused of abducting and decapitating hostages, including American journalist James Foley. — AFP