President Recep Tayyip Erdogan is furious, and he wants the United States to know it.
Media reports are citing US officials who say they have plans to establish a so-called "border force" in northern Syria, using Kurdish-led fighters.
That announcement hasn't gone down well with Turkey.
The 30,000-strong force will patrol the Turkish and Iraqi borders, as well as along the Euphrates River in Syria.
It will include members of the Syrian Democratic Forces, a loose coalition of fighters dominated by the People's Protection Units, the YPG.
Turkish authorities consider the YPG a terrorist organisation, linked to the banned Kurdistan Workers' Party, the PKK, currently waging its own battle for independence within Turkey.
Throughout the Syrian conflict, the US and Turkey have largely worked in cooperation with each other.
The US has led an international coalition battling I-S militants in Syria since 2014, and currently has around 2,000 troops stationed in the country.
Syrian state media has denounced the US border force plan as a "blatant assault" on its sovereignty, saying any Syrian who joined the force would be a "traitor".
Syria's ally, Russia, has also joined the criticism.
Russian Foreign Minister Sergei Lavrov accuses the US government of attempting to exert more control over Syria and, possibly, split it up.
