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".
The coalition said on Monday that its goal was to create a force with about 30,000 personnel "over the next several years". About half will be Kurdish and Arab SDF fighters and the other half new recruits.
The BSF will be tasked with securing the long sections of Syria's northern border with Turkey and eastern border with Iraq that are under SDF control, as well as parts of the Euphrates river valley, which effectively serves as the dividing line between the SDF and Syrian pro-government forces.
"A strong border security force will prohibit Daesh's freedom of movement and deny the transportation of illicit materials," the coalition said, using a different term for IS. "This will enable the Syrian people to establish effective local, representative governance and reclaim their land."
Why is Turkey concerned?
Turkey has consistently opposed the coalition's support for the SDF because the force is dominated by the Kurdish People's Protection Units (YPG) militia.
Ankara considers the YPG an extension of the banned Kurdistan Workers' Party (PKK), which has fought for Kurdish autonomy in Turkey for three decades. Washington disagrees and insists the YPG has been vital to the battle against IS.
On Monday, Turkish President Recep Tayyip Erdogan said the US had acknowledged it was "in the process of creating a terror army on our border".
"It is for us to suffocate this terror army before it is born," he said.
Mr Erdogan added that preparations were complete for a Turkish military operation against the Kurdish enclave of Afrin in north-western Syria, and that it might start "at any moment". Troops deployed at the border were already hitting YPG positions inside Afrin with heavy artillery, he noted.
What do other countries say?
The Syrian government called the creation of the SDF border force "a blatant attack on the sovereignty and territorial integrity and unity of Syria, and a flagrant violation of international law".
"What the American administration has done comes in the context of its destructive policy in the region to fragment countries... and impedes any solutions to the crises," an official at the foreign ministry was cited as saying by the Sana news agency.
The source warned that Damascus considered any Syrian fighting for militias sponsored by the US to be "a traitor to their people and nation".
Russia, which backs the Syrian government, said the US move might lead to the "break-up of a large territory along the border with Turkey and Iraq". "This is a very serious issue that raises concerns that a path towards the partition of Syria has been taken," Foreign Minister Sergei Lavrov said.