The Syrian Foreign Ministry said on Thursday that a U.S military presence in Syria was illegitimate and represented an "aggression" against Syrian sovereignty, after Washington signalled that its forces would stay in parts of Syria indefinitely.
U.S. Secretary of State Rex Tillerson on Wednesday signaled an open-ended military presence as part of a broader strategy to prevent Islamic State's resurgence, pave the way diplomatically for the eventual departure of Syrian President Bashar Assad and curtail Iran's influence.
Turkey added on Thursday Washington's denial that it intended to build a border force in Syria was "important", but added Ankara would not remain silent in the face of any force that threatened its borders.
Turkey reacted angrily after the U.S.-led coalition said it would help its Syrian militia allies, dominated by the Kurdish YPG, to set up a new 30,000-strong border force.
Turkey also said on Thursday Russia should not oppose its operation in Syria's Afrin and will coordinate with Moscow on the situation of Russian observers there.
Foreign Minister Mevlut Cavusoglu also told broadcaster CNN Turk that Turkey will coordinate with Russia and Iran on an air operation in Afrin.