Turkish forces, together with the rebel Free Syrian Army, will cross the Syrian border "shortly," President Tayyip Erdogan's communications director said early on Wednesday after a surprise withdrawal of U.S. troops made way for the Turkish incursion.

Turkey has been poised to advance into northeast Syria since the U.S. troops began vacating the area in an abrupt policy shift by U.S. President Donald Trump that was widely criticized in Washington as a betrayal of America's Kurdish allies.

On Tuesday, Turkish officials said the military had struck the Syrian-Iraqi border to prevent Kurdish forces from using the route to reinforce the region, though details of the strikes were hazy.