Syria on Monday denounced the US-led coalition's plan to create a 30,000-strong border force in the country's northeast, saying it would consider its members "traitors".
The alliance fighting the Islamic State group announced on Sunday that it was working with Arab and Kurdish fighters to establish a Border Security Force (BSF).
The BSF would be responsible for preventing a "resurgence" of IS in areas where the jihadists had been cleared by the Syrian Democratic Forces.
But an official source in Syria's foreign ministry on Monday denounced the plan.
"Syria strongly condemns the US announcement on the creation of militias in the country's northeast, which represents a blatant attack on the sovereignty and territorial integrity and unity of Syria, and a flagrant violation of international law," said the source, cited by state news agency SANA.
"Syria considers any Syrian who participates in these militias sponsored by the Americans as a traitor to their people and nation, and will deal with them on this basis."
Backed by the US-led coalition's air strikes, advisers and weapons, the SDF has ousted IS from swathes of territory in the east and north, including IS bastion Raqa.
With the offensive winding down, the coalition and SDF said they were shifting their focus to border security to prevent a jihadist comeback.
"A strong Border Security Force will prohibit Daesh's freedom of movement and deny the transportation of illicit materials," the coalition said, using the Arabic acronym for IS.
In a new emailed statement on Monday, it said it aimed to create the 30,000-strong force "over the next several years".
About half would be SDF veterans, and another 15,000 would be new recruits.
"The Border Security Force will be stationed along the borders of SDF-held areas, to include portions of the Euphrates river valley and international borders to the east and north of SDF-liberated territory," the coalition said.
On Monday, for the third straight day, Turkish artillery targeted Kurdish positions in the Afrin region of northern Syria, the Syrian Observatory for Human Rights said.
Turkey has reacted sharply to news of the border force, saying it would "legitimise a terror organisation".
Ankara is fiercely opposed to the SDF, which is dominated by the Kurdish People's Protection Units (YPG) -- considered by the Turkish government to be a "terrorist" group.
Both the US-led coalition and the SDF declined to comment on potential rules of engagement with Turkish or Syrian troops.