Washington: The Trump administration has told Turkey it will move to rein in Kurdish fighters that have been the backbone of the US campaign against the Islamic State in Syria, in an effort to repair tattered relations.

The first step and "the crux of the matter", a senior Turkish official said, is to withdraw the Kurds from the Syrian town of Manbij and relocate them east of the Euphrates River. The town, about 40 kilometres from Turkey's border, has come to symbolise the fevered competition for territory and influence in northern Syria among the United States, Turkey, and other regional powers.

The American pledge, if carried out, would satisfy a long-standing demand by the Turkish government and fulfil a promise first made by the Obama administration to keep the Kurdish forces east of the Euphrates. The Kurds helped to take Manbij from the Islamic State in 2016 and have been there since.