US forces withdrew from the border areas with Turkey (Representational image)

US forces in northern Syria started pulling back Monday from areas along the Turkish border ahead of a feared military invasion by Ankara that Kurdish forces say would spark a jihadist resurgence.

The Kurdish-led Syrian Democratic Forces said in a statement that "US forces withdrew from the border areas with Turkey" in northeast Syria.

The Syrian Observatory for Human Rights war monitor confirmed that US forces had pulled back from key positions in Ras al-Ain and Tal Abyad.

The White House on Sunday said that Turkey would "soon be moving forward with its long-planned operation".

It said US troops in northern Syria would no longer be "in the immediate area" near the border with Turkey, nor would they support Ankara's operations.

On Monday, the US-backed SDF said such an operation would reverse years of successful Kurdish-led operations to defeat the Islamic State group and allow some of its surviving leaders to come out of hiding.

It also warned that a Turkish invasion would pose a threat to SDF-run prisons and informal settlements housing thousands of IS jihadists and their families.

Ankara wants to push the US-backed Syrian Kurdish forces from its border, saying that the group is a "terrorist" offshoot of Kurdish insurgents in Turkey.

The Turkish military has twice launched offensives in Syria -- against IS in 2016, and in 2018 against the People's Protection Units (YPG), the backbone of the SDF.

Long marginalised, Syria's Kurds have -- beyond heavy campaigns against IS -- essentially stayed out of the country's eight-year civil war, instead setting up their own institutions in areas under their control.

In the areas of Ras al-Ain, Tal Abyad and Kobane, all bordering Turkey, Kurdish forces have dug trenches and tunnels in preparation for a Turkish offensive, the Observatory said Sunday.