President Recep Tayyip Erdogan also warned US troops against coming between Turkish troops and Kurdish forces, which Ankara views as an extension of Turkey's own Kurdish insurgency.
Turkey has been threatening to launch a new military operation against the main Syrian Kurdish militia, known as the People's Defense Units, or YPG, in the Kurdish-held Afrin enclave in northern Syria.
Russia has also warned that the nascent US force threatens to fuel tensions around Afrin.
"The United States has admitted that it has created a terrorist force along our country's border. Our duty is to drown this terror force before it is born," Erdogan said in a speech in Ankara.
Turkey sent troops into Syria in 2016 to prevent Syrian Kurdish militants from forming a contiguous entity along its border.
In recent days, Turkey said it would soon launch a new operation in Afrin and sent reinforcements to the border. Russia deployed military observers to Afrin last year in an effort to prevent Turkish-Kurdish clashes.
On Monday, Erdogan said preparations for the military assault on Afrin "are complete," adding that an operation could start any moment. He said Turkish troops are already firing artillery at Afrin from the border.
"Don't stand between us and these herd of murderers. Otherwise, we won't be responsible for the unwanted incidents that may arise," he said. "Tear off the insignia you have placed on the uniforms of the terrorists so that we don't have to bury them (US soldiers) together with the terrorists.
Russia said Monday that the new force is a sign Washington "doesn't want to preserve the territorial integrity of Syria." Foreign Minister Sergey Lavrov said the nascent border force is "not helping calm the situation."
The Syrian government also condemned the US plans for the border force, saying it would consider its members "traitors".
"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 Syria Foreign Ministry.
"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."
AP and AFP contributed to this story.