Syrian Kurds on Wednesday called on civilians to help defend their territory against Turkey, after the Turkish forces said they were poised to cross into Syria "shortly".

The Kurdish administration in northeastern Syria asked the residents to defend the region against a Turkish assault believed to be imminent, according to the Agence France-Presse.

"We announce three days of general mobilization in northern and eastern Syria," the administration said in a statement, urging all civilians to "head to the border with Turkey to fulfill their duty".

The local authority, the Autonomous Administration of North and East Syria, called on the international community to live up to its responsibilities as "a humanitarian catastrophe might befall our people" in northeastern Syria.

US President Donald Trump abruptly announced on Sunday that the US military "will not support or be involved in" an expected military operation by Turkey in northern Syria, and US forces "would no longer be in the immediate area".

Fahrettin Altun, communication director for Turkish President Recep Tayyip Erdogan, said on Wednesday that fighters from the Kurds' People's Protection Units, or YPG, could either defect to Turkey or Ankara would have to "stop them from disrupting" Turkey's struggle against the Islamic State militants, Reuters reported.

"Turkey has no ambition in northeastern Syria except to neutralize a long-standing threat against Turkish citizens and to liberate the local population from the yoke of armed thugs," Altun wrote in a Tuesday column for The Washington Post.

Turkey's Demiroren news agency said the Free Syrian Army, a Syrian rebel group, traveled from northwestern Syria to Turkey in preparation for the incursion in Syria's northeast.

Meanwhile, the Syrian Democratic Forces, or SDF, led by the YPG, said Turkish forces were already attacking near the border.

The Syrian Kurdish fighters are viewed as terrorists by Turkey as they are affiliated with Turkey's outlawed Kurdistan Workers' Party, which has waged a 35-year-long battle for autonomy within Turkey.

The SDF forces control the east of the Euphrates River in northern Syria, and have been a key partner of the United States in the fight against the Islamic State group.

Li Weijian, a researcher at Shanghai Institutes for International Studies, said: "Ankara is afraid that the Kurds in Turkey will connect with the Syrian Kurds."

"The Turks will definitely seize the opportunity to cut the tie between the two groups of Kurds, because the US withdrawal from Syria left the issue to Turkey."

On Monday, Erdogan said US troops had started to withdraw after a phone call he had with Trump, adding that talks between Turkish and US officials on the matter would continue.

In Moscow, Russia Foreign Minister Sergei Lavrov said on Wednesday that Syria's territorial integrity must be preserved, Reuters reported.