BEIRUT/ISTANBUL - 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.

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

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.

The Syrian Observatory for Human Rights monitoring group said Turkish forces and Syrian rebels entered Suluk, some 10 km from Turkey's border. Turkey's state-owned Anadolu news agency said the rebels seized complete control of Suluk.

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 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 Kurdish-led Syrian Democratic Forces (SDF), in which the YPG comprises the main fighting element, had recovered "almost full control" of Ras al Ain after a counter attack.

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

Turkey's incursion has raised international alarm over its mass displacement of civilians and the possibility of Islamic State militants escaping from Kurdish prisons. The Kurdish-led forces have been key allies for the United States in eliminating the jihadist group from northern Syria.

More than 130,000 people have been displaced from rural areas around Tel Abyad and Ras al Ain as a result of the fighting, the United Nations said on Sunday.

In a statement, the U.N. Office for the Coordination of Humanitarian Affairs said OCHA and other relief agencies estimated up to 400,000 civilians in the Syrian conflict zone may require aid and protection in the coming period.

In the latest criticism, British Prime Minister Boris Johnson expressed "grave concern" to Turkish President Tayyip Erdogan, saying the offensive 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 a telephone call between the two leaders on Saturday evening. -Reuters