Turkey’s president has denounced US plans to form a 30,000-strong Kurdish-led border security force in Syria, vowing to "drown this army of terror before it is born".
Russia and Syria have also rejected the idea.
Turkish president Recep Tayyip Erdogan also warned US troops against coming between Turkish soldiers and Kurdish forces, which Ankara view 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 Defence Units, or YPG, in the Kurdish-held Afrin enclave in northern Syria.
The YPG is the backbone of a Syrian force that drove the Islamic State group from much of northern and eastern Syria with the help of US-led airstrikes.
Russia has also warned that the nascent US force threatens to fuel tensions around Afrin.
Mr Erdogan said in a speech in Ankara: "The United States has admitted that it has created a terrorist force along our country’s border. Our duty is to drown this army of terror before it is born."
The US-led coalition said: "Turkey is a valued member of a 74-member Coalition and a Nato partner, sharing our mission to ensure the lasting defeat of (IS) in Iraq and Syria. It would be inappropriate for us to comment on Mr Erdogan’s remarks."
The coalition said the new force, expected to reach 30,000 in the next several years, is a key element of its strategy in Syria to prevent the resurgence of the IS group in Syria.
The SDF currently controls nearly 25% of Syrian territory in the north and east. The core of the force is to be made up of fighters from the existing Kurdish-led Syrian Democratic Forces, the coalition’s ally in the fight against IS.
Some 230 cadets have already been recruited to the new border force, according to the coalition. The force is expected to be deployed along the borders of the SDF-held areas and Iraq and Turkey.
Turkey sent troops into Syria in 2016 to prevent Syrian Kurdish fighters from forming a contiguous entity along its border. It has also supported rival Syrian rebels and independently fought to drive IS from parts of Syria.
Tensions with Washington have repeatedly erupted over its support of the SDF, prompting US troops to deploy in north-east Syria to prevent clashes between the Kurdish forces and Turkey-backed fighters.
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 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".
Bashar Assad’s government also condemned the US plans for the border force, calling it "a blatant encroachment upon the sovereignty, unity and territorial integrity of Syria" and a violation of international law.
Syrian Kurdish leader hopes new force will counter threats from Turkey, Damascus https://t.co/sf8PQm75gz— Reuters World (@ReutersWorld) January 15, 2018
- Press Association