Turkish president Recep Tayyip Erdogan has confirmed that US troops have started withdrawing from positions in northern Syria.
Mr Erdogan spoke hours after the White House said American forces in north-east Syria will move aside and clear the way for an expected Turkish incursion.
The announcement has cast uncertainty on the fate of Kurdish fighters allied with the US against the so-called Islamic State group (IS).
Turkey has threatened for months to launch a military operation to drive away Syrian Kurdish fighters from a border region east of the Euphrates River.
Mr Erdogan said Turkey is determined to halt what it perceives as threats from the Syrian Kurdish fighters.
He also said he planned to travel to Washington next month to meet with US president Donald Trump.
The US move essentially abandons Kurdish fighters who fought alongside American forces in the years-long battle to defeat IS.
The Syrian Democratic Forces (SDF) said the move comes as Turkey is preparing to attack Kurdish-held areas in north-east Syria.
The group’s statement warns the Turkish invasion would be a blow to the fight against IS militants.
The Kurdish Hawar news agency and the UK-based Syrian Observatory for Human Rights also said American troops were evacuating positions near the towns of Ras al-Ayn and Tal Abyad on Monday.
The Syrian Kurdish fighters also accused Washington of failing to abide by its commitments to its key allies in the fight against IS.
The move marks a major shift in US policy. There has been no immediate comment on the pullout from US officials.
Mr Erdogan has threatened for months to launch the military operation across the border. He views the Syrian Kurdish forces as a threat to his country, as Ankara struggles with a Kurdish insurgency within Turkey.
In the US, Republicans and Democrats have warned that allowing the Turkish attack could lead to a massacre of the Kurds and send a troubling message to American allies across the globe.
The SDF said: “The American forces did not abide by their commitments and withdrew their forces along the border with Turkey.
“Turkey now is preparing to invade northern and eastern parts of Syria.”
In March, the group captured the last sliver of land held by the extremists, marking the end of the so-called caliphate that was declared by IS’s leader Abu Bakr al-Baghdadi in 2014.
The SDF said: “We will not hesitate for a moment in defending our people” against Turkish troops, adding that it has lost 11,000 fighters in the war against IS in Syria.
A Turkish attack would lead to a resurgence of IS, it said. IS sleeper cells are already plotting to break free some 12,000 militants detained by Syrian Kurdish fighters in north-eastern Syria in a “threat to local and international security”.
The Kurdish fighters also control the al-Hol camp, home to more than 70,000 mostly wives and children of IS fighters.
Turkish foreign minister Mevlut Cavusoglu tweeted that since the beginning of the crisis in Syria, “we have supported the territorial integrity of this country, and we will continue to support it”.
He added that Ankara is determined to ensure the survival and security of Turkey “by clearing the region from terrorists. We will contribute to peace, peace and stability in Syria”.