ANKARA (BLOOMBERG) - Turkish troops have begun crossing into north-eastern Syria to force back Kurdish militants controlling the border area, a Turkish official told Bloomberg, days after President Donald Trump said the US wouldn't stand in the way.

A small forward group of Turkish forces entered Syria early on Wednesday (Oct 9) at two points along the frontier, close to the Syrian towns of Tal Abyad and Ras al-Ayn, in preparation for the broader offensive, said the official, who spoke on condition of anonymity.

There was no immediate comment from Kurdish-led forces, who vowed earlier in a televised news conference from Syria's northern city of Hasake to defend themselves against any Turkish offensive.

Defence Minister Hulusi Akar said "deployments and work is still underway regarding the operation", according to state-run TRT television.

In a dramatic reversal of US policy, Trump told Turkey's President Recep Tayyip Erdogan in a phone call on Sunday that dozens of American troops who had been working closely with Kurdish-led forces in the fight against Islamic State would pull back, effectively clearing the way for a Turkish advance.