Key powers involved in Syria's civil war have criticised US plans to help set up a 30,000-strong Syrian Kurdish "border security force."
Turkey's president vowed to "suffocate" efforts to begin training members of the Syrian Democratic Forces.
"Now the US has acknowledged it has established an army of terror along our country's border. It's up to us to drown this army of terror before it is born," said Turkish President Recep Tayyip Erdogan.
"Don't stand between us and a herd of murderers. Otherwise, we will not be responsible for unwanted incidents."
Ankara considers Kurds fighting for the SDF to be part of a terrorist group.
The force will be tasked with securing Syria's northern border with Turkey and the eastern border with Iraq.
Syria's government has already decried what it call a "blatant attack" on its sovereignty, and Russia has warned that it could lead to separation.
"This is a very serious issue, which causes concerns that a course was set for the partition of Syria." said Russian foreign minister, Sergei lavrov
With the help of air strikes from a US-led coalition, the SDF now currently controls nearly 25 percent of Syria's territory along the borders with Turkey and Iraq, captured from Islamic State (IS) militants.