US President Donald Trump has ordered all remaining US forces in north-east Syria to withdraw as the military confrontation between Turkey and Kurdish fighters escalates, Defense Secretary Mike Esper said on Sunday.

Esper, who spoke during two TV interviews, did not say whether the US troops would leave Syria completely or where they would go.

According to Esper, Trump's national security team was scheduled to meet later on Sunday to assess the situation in Syria.

US forces had been collaborating for about five years with a Kurdish-led Syrian group called the Syrian Democratic Forces to fight the Islamic State group and its attempts to establish a caliphate in the region.

Turkey, however, never approved of the alliance as it considers parts of that force to be terrorists linked to an insurgency inside Turkey.

Turkey advanced into northern Syria on Wednesday after President Trump ordered American troops last week to pull out of the area.

Meanwhile, at least 26 civilians were killed on Sunday in northeastern Syria as Turkey stepped up its offensive against Kurdish forces, according to the Syrian Observatory for Human Rights.

This brings the number of civilians killed since the start of the incursion to nearly 50.

Over 70,000 people have been displaced inside Syria according to UN officials.