Turkey's President has vowed to "drown" a 30,000-strong Syrian Kurdish border security force the United States is setting up.
Recep Tayyip Erdogan accused the US, a Nato ally of Turkey, of forming a "terrorist" force on his country's border.
Ankara considers the Syrian Kurdish militia a terror group, claiming it is affiliated with Turkey's own outlawed Kurdish rebels who are fighting against the government.
"A country we call an ally is insisting on forming a terror army on our borders," Mr Erdogan said in a speech in Ankara.
"What can that terror army target but Turkey?"
He also said Turkey's military at the border with Syria was striking Syrian Kurdish fighters with heavy artillery, "at the moment hitting them with howitzers and we will continue to strike them."
Last week, the US-led coalition said it was working with the mainly Kurdish YPG-led Syrian Democratic Forces (SDF) to set up a 30,000-stronb border force.
The plan infuriated Turkey, which considers the YPG to be an extension of the PKK, a Kurdish group waging an insurgency in southern Turkey and deemed a terrorist organisation by Turkey, the S and European Union.
"This is what we have to say to all our allies: don't get in between us and terrorist organisations, or we will not be responsible for the unwanted consequences," Mr Erdogan added.
"Either you take off your flags on those terrorist organisations, or we will have to hand those flags over to you... Our operations will continue until not a single terrorist remains along our borders, let alone 30,000 of them."
The Syrian regime of Bashar al-Assad responded to the US plan by vowing to crush the new force and drive US troops from Syria.
Russia, the Syrian dictator's ally, called the plans a plot to dismember Syria and place part of it under US control.
The US has led an international coalition using air strikes and special forces troops to aid fighters on the ground battling Isis militants in Syria since 2014. It has about 2,000 troops on the ground in Syria.
Washington says its troops are prepared to stay to make sure the Islamist militant group cannot return, and also cited the need for meaningful progress in UN-led peace talks.
For much of the war, the United States and Turkey worked together, jointly supporting forces fighting against the Assad government. But a US decision to back Kurdish fighters in northern Syria in recent years has enraged Ankara.
