Turkish forces and their Syrian allies seized large parts of the northern Syrian town of Suluk, a war monitor said on Sunday, as they pressed on with their offensive against Kurdish militia for a fifth day in the face of fierce international opposition.

The Kurdish-led Syrian Democratic Forces (SDF) accused Turkey-backed rebel fighters of executing 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 monitoring group said "nine civilians were executed at different moments south of the town of Tel Abyad" by Turkey-backed groups, including Hevrin Khalaf, co-chair of the secular Future Syria Party.

The Kurdish-led Syrian Democratic Forces said 31 of its fighters had been killed fighting a Turkish offensive in northern Syria since Saturday, increasing the SDF's official death toll to 76.

Turkey is facing threats of possible sanctions from the United States unless it calls off the incursion, while the Arab League has denounced the operation and NATO allies Germany and France said they were halting weapons exports to Turkey.

Ankara launched the cross-border assault against the YPG militia after U.S President Donald Trump withdrew some U.S. troops from the border region. Turkey says the YPG is a terrorist group aligned with Kurdish militants in Turkey.

Open gallery view Hevrin Khalaf. Credit: Future Syria Party

Funeral held for Kurdish politician purportedly killed by Turkey-backed fighters in Syria

A U.S. official says the situation in northeast Syria is "deteriorating rapidly" as Turkey-backed forces advance and could isolate American forces on the ground.

The official said Sunday the development is quickly increasing the risk of a confrontation between Turkey-backed and U.S. forces in the area. The official spoke to The Associated Press on condition of anonymity because they were not authorized to brief reporters

The Syrian Observatory said Turkish forces and Syrian rebels entered Suluk, some 10 km (6 miles) from Turkey’s border. Turkey’s state-owned Anadolu news agency said the rebels seized complete control of Suluk.

Open gallery view A Turkish-baked Syrian fighter fires during clashes in the Syrian border town of Ras al-Ain, October 13, 2019. Credit: AFP

Suluk is southeast of the Syrian border town of Tel Abyad, one of the two main targets in the incursion, which was shelled by Turkish howitzers on Sunday morning, a witness in the neighboring Turkish town of Akcakale said.

Gunfire also resounded around the Syrian border town of Ras al Ain, some 120 km (75 miles) to the east of Tel Abyad, while Turkish artillery continued to target the area, a Reuters reporter across the border in Turkey’s Ceylanpinar said.

Turkish-backed Syrian rebels, known as the National Army, advanced into Ras al Ain on Saturday but by Sunday there were still conflicting reports on who held control.

The Syrian Observatory monitoring group said the SDF, in which the YPG comprises the main fighting element, had recovered “almost full control” of Ras al Ain after a counter attack.

Open gallery view Smoke rises from an explosion during an offensive by Turkish-led forces in Tel Abyad, Syria, October 13, 2019. Credit: Reuters

A spokesman for the National Army denied this, saying its forces were still in the positions they took on Saturday.

In the latest criticism, British Prime Minister Boris Johnson expressed “grave concern” about the offensive to Turkish President Tayyip Erdogan, saying it may worsen the humanitarian situation and undermine progress against Islamic State.

“He urged the president to end the operation and enter into dialogue,” a spokesman for Johnson said after the telephone call between the two leaders on Saturday evening.

Turkey’s Defence Ministry said on Sunday that 480 YPG militants had been “neutralised” since the operation began, a term that commonly means killed.

The Syrian Observatory for Human Rights, a UK-based organization which reports on the war, said 74 Kurdish-led fighters, 49 Turkey-backed Syrian rebels and 30 civilians have been killed in the fighting.

Open gallery view Kurdish communities in Syria, 2019. Credit: Reuters

In Turkey, 18 civilians have been killed in cross-border bombardment, Turkish media and officials say.

Trump Defends decision to pull out U.S. forces from Syria

Trump on Saturday defended his decision to withdraw troops from the Syrian border region, telling conservative Christian activists that the United States should prioritize protecting its own borders.

“Let them have their borders, but I don’t think our soldiers should be there for the next 50 years guarding a border between Turkey and Syria when we can’t guard our own borders at home,” Trump said in a speech in Washington.

“Don’t forget: they are fighting for their land. They haven’t help us fight for our land,” Trump said. “They’re fighting for their land and that’s good, but we’ve helped them.”

The Kurdish-led administration in Syria’s northeast has said nearly 200,000 people had been uprooted so far by the fighting, while the UN World Food Program said more than 130,000 had left Ras al Ain and Tel Abyad.

The water situation in the north-eastern city of al-Hassakeh and its surroundings is rapidly deteriorating, the UN Office for the Coordination of Humanitarian Affairs (OCHA) added.

Open gallery view Areas of control in Syria. Credit: Reuters

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.

He has also dismissed the growing condemnation of the operation, saying that Turkey “will not stop it, no matter what anyone says”.

The Kurdish-led Syrian Democratic Forces (SDF), in which the YPG comprises the main fighting element, holds most of the northern Syrian territory that once made up ISIS' “caliphate” in the country.

The SDF has been keeping thousands of fighters from the jihadist group in jail and tens of thousands of their family members in camps. Islamic State has claimed responsibility for a car bomb on Friday in Qamishli, the largest city in the Kurdish-held area, where some ISIS militants fled from a jail.