Key powers involved in Syria's civil war have criticised US plans to help an allied Kurdish-led militia set up a 30,000-strong "border security force".
Turkey's president vowed to "suffocate" efforts to begin training members of the Syrian Democratic Forces (SDF) and create what he called a "terror army".
Ankara considers Kurds fighting for the SDF to be part of a terrorist group.
Syria's government decried the "blatant attack" on its sovereignty, and Russia warned it could lead to partition.
With the help of air strikes from a US-led coalition, the SDF has captured tens of thousands of square kilometres of territory from Islamic State (IS) militants.
In October, the alliance took full control of the northern city of Raqqa, the de facto capital of the "caliphate" declared by the jihadist group in 2014. Since then, SDF fighters have been advancing south-eastwards along the Euphrates river valley.
Why is the US creating the border force?
News of the coalition's plan to work with the SDF to train a new Syrian Border Security Force (BSF) was first reported on Saturday by The Defense Post, which quoted a spokesman as saying that 230 individuals were currently participating in the "inaugural class".