Turkey has begun bolstering its military presence in Kilis near near its border with Syria, a war monitor said.
The Turkish IHA news agency reported that a convoy of Turkish troops had been sent into Syria overnight.
The Syrian Observatory for Human Rights said the reinforcements were sent to the front line with Manbij, where U.S. troops have been based. According to the Observatory, 50 vehicles crossed into Syria - carrying troops and equipment.
A Turkish military official said the military reinforcements were dispatched to the areas administered by Turkey in northern Syria.
The Turkish buildup comes even though Turkey said it would delay a promised offensive in eastern Syria in the wake of U.S. President Donald Trump's decision to withdraw his country's troops earlier this week.