The US-backed Syrian Democratic Forces (SDF) warned Turkey that a military offensive against them would imminently cause a resurgence of Daesh* in the region.
The Kurdish-led militants also claimed that US forces have begun withdrawing from the areas near the Turkish border.
"The American forces did not fulfill their commitments and withdrew their forces from the border areas with Turkey, and Turkey is now preparing for an invasion operation of northern and eastern Syria", the SDF said.
The Kurdish-led forces also stated they "won't hesitate for a moment" in defending themselves against Turkey. Later in the day, an SDF spokesman claimed that the move by the US military was a "stab in the back" for the militants.
"There were assurances from the United States of America that it would not allow any Turkish military operations against the region," SDF spokesman Kino Gabriel said in an interview with al-Hadath TV.
Commenting on the operation, the Kremlin said that Erdogan hasn't discussed the plans with Russian President Vladimir Putin and stressed that other countries should respect Syria's territorial integrity.
US President Donald Trump proposed the idea of establishing a buffer-zone to resolve tensions between Ankara and the SDF militia in January after his Turkish counterpart Recep Tayyip Erdogan had promised to start an operation against the Kurdish-led forces in Syria. Both sides agreed on the scenario, but in September Ankara accused Washington of stalling the process and promised to drive the militants out of the area, and create the zone by itself, if necessary.
The Syrian government has decried the safe zone deal between Turkey and the US, calling both countries occupiers, as they operate in Syria without the permission of Damascus.
According to Turkish authorities, the SDF, led by the Kurdish People's Protection Units, is connected to the Kurdistan Workers' Party (PKK), blacklisted in the country as a terror group. In 2018, Turkey launched a military operation in the then-Kurdish controlled Syrian city of Afrin, claiming that the militants posed a threat to the country.
The SDF engaged in battles against Daesh terrorists across Syria over the past few years but also opposed the government in Damascus.
* Daesh (ISIL/ISIS/IS/Islamic State) is a terrorist organisation banned in Russia