Turkey began reinforcing its positions on both sides of its border with Syria on Sunday, a Reuters witness said, as Ankara and Washington agreed to coordinate a US withdrawal from Syria.
The heightened military activity comes two days after President Tayyip Erdogan said Turkey would postpone a planned military operation on Kurdish YPG militia east of the Euphrates river in northern Syria following the US decision to pull out.
He also said Turkey would take over the fight against Daesh militants.
The Turkish presidency said Erdogan and US President Donald Trump in a phone call on Sunday agreed to establish military and diplomatic coordination to prevent an authority vacuum from developing as the United States withdraws.
Earlier in the day, footage from broadcaster TRT World showed some Turkish convoys entering Syria via the Turkish border town of Karkamis, which is located some 35 km (22 miles) north of the northern Syrian town of Manbij.
The convoys are crossing into an area controlled by the Free Syrian Army (FSA), a Turkish ally, and are heading to the frontlines of Manbij, TRT World said.
Manbij has been a major flashpoint between Ankara and Washington. In June, the NATO allies reached an agreement that would see the YPG ousted from the town but Turkey has complained the roadmap has been delayed and said Turkish forces would enter the town if the United States does not remove the Kurdish militants.
The YPG has been a main US ally in the fight against Daesh. Ankara says the militia is a terrorist organisation and an extension of the Kurdistan Workers Party (PKK), which has waged an insurgency in Turkey since the 1980s.