Turkey defied growing condemnation from its NATO allies to press ahead with its invasion of northern Syria on Tuesday, shelling suspected Kurdish positions near the border amid reports that Syrian Kurds had retaken a key town.

Targeting Turkey's economy, US President Donald Trump on Monday announced sanctions aimed at restraining the Turks' assault against Kurdish fighters and civilians in Syria - an assault Turkey began after Trump announced he was moving US troops out of the way.

Now in its seventh day, Turkey's offensive has sowed fear and chaos in an already war-weary region - and upended alliances amid Syria's eight-year conflict.

The Associated Press reported heavy bombardment of targets in the countryside of Ras al-Ayn early on Tuesday, days after Turkey announced that it had captured the border town. Turkish jets also carried out at least one airstrike.