The number of Turkish military patrols on the frontier with armoured vehicles "increased" in the border town of Akcakale, the private DHA news agency said.

Nine trucks carrying armoured vehicles and a bus with military personnel on board had reached Akcakale in Sanliurfa province, state news agency Anadolu reported late Saturday.

The convoy was sent to reinforce the military units on the Syria border, the agency said, without directly linking the arrival with the operation threatened by Turkey.

Erdogan on Saturday renewed his vow to start an "air and ground" offensive against the Kurdish People's Protection Units (YPG) militia, viewed as a terrorist group by Ankara.

He said preparations had been completed for an operation that he warned could be as soon as "today, tomorrow". Turkey has previously said plans for the offensive were in place.

Akcakale is across from the Syrian town of Tal Abyad which was captured by the YPG from the Islamic State extremist group in 2015.

Kurdish authorities said in August their forces had started withdrawing from Tal Abyad.

Although the United States and Turkey agreed a deal in August to establish a buffer zone in northern Syria to keep the militia away from the Turkish border, Ankara has repeatedly expressed dissatisfaction with the slow progress of any "safe zone".