Turkey began its onslaught against YPG militia that it says is a terrorist group

Turkish-backed Syrian rebels advanced into the border town of Ras al Ain in northeast Syria on Saturday but it was unclear how far, with Turkey saying it had taken the town center, and Kurdish-led forces denying that and saying they were counter-attacking.

Turkey pursued its four-day-old, cross-border offensive against a Syrian Kurdish militia despite an outcry from the United States and European Union and warnings of possible sanctions unless Ankara called off its attack.

U.S. President Donald Trump's administration said Turkey's incursion was causing "great harm" in relations with its NATO ally. Other NATO allies Germany and France said they were banning weapons exports to Turkey. The head of the Arab League denounced the operation and British Prime Minister Boris Johnson spoke to Turkish President Tayyip Erdogan to express his "grave concern" about the offensive.

Ankara began its onslaught against the YPG militia, which it says is a terrorist group backing Kurdish rebels in Turkey, after Trump withdrew some U.S. troops there to support Kurdish forces in the fight against Islamic State.

The assault has raised international alarm over its mass displacement of civilians and the possibility of Islamic State militants escaping from Kurdish prisons leading to a revival of the jihadist group's insurgency in Syria.

The Kurdish-led administration in Syria's northeast said nearly 200,000 people had been uprooted so far by the fighting, while the U.N. World Food Programme said more than 100,000 had left Ras al Ain and the town of Tel Abyad.

Turkey's stated objective is to set up a "safe zone" inside Syria to resettle many of the 3.6 million Syrian war refugees it has been hosting. Erdogan has threatened to send them to Europe if the EU does not back his assault.

Turkish officials posted photos on Saturday showing deserted streets and Turkish-backed Syrian rebels standing on Kurdish militia flags in Ras al Ain.

"The (Syrian rebel) National Army took control of (Ras al Ain) town center this morning," a senior Turkish security official said, referring to Ankara-backed rebels. "Inspections are being conducted in residential areas. Mine and booby trap searches are being carried out."

Marvan Qamishlo, a spokesman for the Kurdish-led Syrian Democratic Forces (SDF), in which the YPG comprises the main fighting element, said they had carried out only a "tactical retreat" in Ras al Ain in response to hours of heavy Turkish bombardment.

"Now the SDF's attack has started and there are very fierce clashes," Qamishlo told Reuters. "The clashes are continuing in the industrial district," the part of Ras al Ain closest to the border.

Speaking as night fell Mustafa Bali, head of the SDF media office, said the SDF remained in control inside Ras al Ain.

The senior Turkish official said "nearly all" YPG forces had fled south from Ras al Ain. Turkish artillery continued to shell parts of the town, a Reuters reporter said.

The SDF holds most of the northern Syrian territory that once made up Islamic State's "caliphate" in the country, and has been keeping thousands of fighters from the jihadist group in jail and tens of thousands of their family members in camps.

The SDF accused Turkey-backed rebel fighters of killing a Kurdish politician in a road ambush on Saturday. The rebel force denied it, saying it had not advanced that far.

The Syrian Observatory for Human Rights, a UK-based organization which reports on the war, said Turkey-backed groups had killed nine civilians on the road, including Hervin Khalaf, co-chair of the secular Future Syria Party.