US forces have been pulling back from positions along the Turkey-Syria border

Turkey will not permit the Islamic State group to return, a presidential spokesman said Monday, amid fears its threatened offensive against a Kurdish militia in Syria could bolster the jihadists.

"Turkey will also continue to fight against DAESH (IS) and will not allow it to return in any shape and form," Ibrahim Kalin said on Twitter.

US forces have been pulling back from positions along the Turkey-Syria border after the White House said Sunday it would not stand in the way of a Turkish operation against Kurdish militants in Syria.

The Kurdish-led Syrian Democratic Forces (SDF) said the US withdrawal threatened to create a security vacuum that would "reverse the successful effort to defeat ISIS".

Ankara says its planned "safe zone" in northern Syria could allow up to two million Syrian refugees to return.

The safe zone "will serve two purposes: secure Turkey's borders by eliminating terrorist elements and allow refugees to return to their homes," Kalin said.

He said Turkey had "no interest in occupation or changing demographics".

There are over 3.6 million Syrian refugees in Turkey, the highest number in the world, which has become an increasing source of tension in the country.