Turkey has begun its military offensive into northeastern Syria to force back Kurdish militants controlling the border area, days after President Donald Trump said the U.S. wouldn't stand in the way.

President Recep Tayyip Erdogan announced the start of the operation, code-named "Peace Spring," on Twitter and said it would also target Islamic State. A small forward group of Turkish forces first entered Syria early Wednesday at two points along the frontier, close to the Syrian towns of Tal Abyad and Ras al-Ayn, according to a Turkish official, who spoke on condition of anonymity.

Kurdish-led forces were on high alert and called on fighters to head for the frontier to defend the region against the Turkish offensive that is expected to involve tens of thousands of soldiers backed by tanks and armored personnel carriers from NATO's second-largest army.