In this photo taken from the Turkish side of the border between Turkey and Syria, in Akcakale, Sanliurfa province, southeastern Turkey, members of the Kurdish People's Protection Units, or YPG, hold a position on a building in the Syrian town of Tel Abyad. Picture: Lefteris Pitarakis/AP

Beirut - The Kurdish-led Syrian Democratic Forces (SDF) declared a three-day state of heightened alert on Wednesday ahead of a planned Turkish incursion into north-eastern Syria.

"As threats and mobilization of the Turkish Army and its mercenaries escalate ... we call on our people ... to go to the area bordering Turkey to carry out their moral duty and show resistance at these sensitive historical moments," an SDF statement said.

Turkey has been sending reinforcements to posts along its border with Syria.

A convoy of more than a 100 vehicles, carrying troops, weapons and construction equipment was headed to Sanliurfa's Akcakale district, some 6 kilometres from the strategic northern Syrian town of Tal Abyad, state news agency Anadolu reported.

"The Turkish military, together with the Free Syrian Army, will cross the Turkish-Syrian border shortly," warned Fahrettin Altun, Turkish President Recep Tayyip Erdogan's communications director, in an overnight Washington Post op-ed.

A Turkish army officer prepares to upload a tank from a truck to its new position on the Turkish side of the border between Turkey and Syria, in Sanliurfa province, southeastern Turkey. Picture: Lefteris Pitarakis/AP

On Monday, US troops started pulling back from areas in north-eastern Syria along Turkey's border, paving the way for a Turkish incursion. 

The SDF, which has been the main partner allied with the US in defeating Islamic State, dominates a swathe of territory in north-eastern Syria.

Ankara has been pushing for the removal of Syrian Kurdish forces along its border, saying they are tied to the outlawed Kurdistan Workers' Party (PKK) waging an insurgency within Turkey.

The SDF also called on Kurds across the world to carry out protests and sit-ins. 

It said the United Nations, the US, the European Union and Russia would be responsible for a "humanitarian disaster" in northern Syria if Turkey invades.

dpa