Russia and Turkey have criticized the United States for helping an allied Kurdish-led militia set up a 30,000-strong border security force in Syria.
"The United States has now acknowledged that it is setting up a terror army along our country's borders," Turkish President Recep Tayyip Erdogan said on January 15.
"What falls on us is to strangle this terror army before it is ever born," he added.
Syria's government called the creation of the border force "a blatant attack on the sovereignty, territorial integrity, and unity of Syria, and a flagrant violation of international law."
Russia, which has given President Bashar al-Assad's government crucial support throughout Syria's civil war, warned it could lead to the "break-up of a large territory along the border with Turkey and Iraq."
"This is a very serious issue that raises concerns that a path toward the partition of Syria has been taken," Foreign Minister Sergei Lavrov said.
The U.S.-led coalition fighting the extremist group Islamic State (IS) said that its goal was to create a border security force with about 30,000 personnel "over the next several years."
The force is to be deployed at the borders of the area controlled by the Syrian Democratic Forces (SDF) -- an alliance of Arab and Kurdish militias dominated by the Kurdish YPG militia, which Ankara considers to be a terrorist group.
The coalition said that about 230 individuals were currently undergoing training in the force's inaugural class.
About half of the force will be SDF veterans, it said.
Backed by the U.S.-led coalition's air strikes, advisers, and weapons, the SDF has ousted IS militants from swaths of northern and eastern Syria.
Its members now control territory bordering Turkey to the north, Iraq to the east, and Syrian government forces to the west.
With reporting by Reuters, AP, dpa, and the BBC