The Turkish President has vowed to "drown" a US-backed border security force with Kurdish fighters in Syria, exposing the fragile state of relations between the two nations.

Recep Tayyip Erdogan accused the US, a Nato ally of Turkey, of forming a "terrorist" force on his country's border that would not be tolerated and would be dealt with "before it is even born".

“A country we call an ally is insisting on forming a terror army on our borders,” Mr Erdogan said of the United States in a speech in Ankara. “What can that terror army target but Turkey?”

Turkish President Erdogan: 'Jerusalem, Mr. Trump, is a red line for Muslims'

On Sunday, the US-led coalition said it was working with its Syrian militia allies, the mainly Kurdish-led Syrian Democratic Forces (SDF), to set up the new border force, whose numbers would be expected to reach 30,000 over a number of years.

The force would patrol along the borders with Turkey and Iraq, as well as within Syria along the Euphrates river, which separates most SDF territory from that held by the government and look to defend territory held by the US-backed Kurdish-led fighters.

The US has led a coalition of nations that has used air strikes and specialist troops to help fighters on the ground battle Isis since the jihadi group took over swathes of territory across Syria and Iraq in 2014. The group has now lost almost all of its territory across the two nations.

The SDF is dominated by the Kurdish People's Protection Units (YPG), and the plan for the force dashes Turkish hopes that the US would abandon the YPG once the war against Isis came to an end. Turkey regards the Kurdish YPG militia as affiliated with Turkey's own outlawed Kurdistan Workers' Party (PKK) who are considered a terror group within Turkey.

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 enraged Ankara.

The new plan offers a glimpse of the strategy of the administration of President Donald Trump, with US forces seemingly wanting to make sure that any possibility of Isis returning is as remote as possible.

President Erdogan was quick to warn the US and other nations over what the future might hold if the plan goes ahead. “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.”

Syrian Democratic Forces take Raqqa from Isis – in pictures Show all 8 1 /8 Syrian Democratic Forces take Raqqa from Isis – in pictures Syrian Democratic Forces take Raqqa from Isis – in pictures Syrian Democratic Forces fighters celebrate victory in Raqqa atop of military vehicles REUTERS Syrian Democratic Forces take Raqqa from Isis – in pictures Fighters of Syrian Democratic Forces march past destroyed buildings as they celebrate victory REUTERS Syrian Democratic Forces take Raqqa from Isis – in pictures Fighters of Syrian Democratic Forces gesture the "V" sign in Raqqa REUTERS Syrian Democratic Forces take Raqqa from Isis – in pictures Fighters of Syrian Democratic Forces evacuate a civilian from the stadium REUTERS Syrian Democratic Forces take Raqqa from Isis – in pictures A civilian prays after she was rescued by fighters of Syrian Democratic Forces from the stadium REUTERS Syrian Democratic Forces take Raqqa from Isis – in pictures Syrian Democratic Forces fighters ride atop of military vehicles as they celebrate victory in Raqqa REUTERS Syrian Democratic Forces take Raqqa from Isis – in pictures Syrian Democratic Forces fighters celebrate victory in Raqqa REUTERS Syrian Democratic Forces take Raqqa from Isis – in pictures A fighter of Syrian Democratic Forces celebrates in Raqqa Reuters

"Either you take off your flags on those terrorist organisations, or we will have to hand those flags over to you," he added. "Our operations will continue until not a single terrorist remains along our borders, let alone 30,000 of them."

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".

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.

Damascus denounced the new border force as a “blatant assault” on its sovereignty, Syrian state media said. It said any Syrian who joined the force would be deemed “a traitor”.

“What the American administration has done comes in the context of its destructive policy in the region to fragment countries ... and impede any solutions to the crises,” state news agency SANA cited a foreign ministry source as saying.

Russia, an ally of the Syria government, called the plans a plot to dismember Syria and place part of it under US control.

Russian Foreign Minister Sergei Lavrov said: “The actions that we see now show that the United States does not want to maintain the territorial integrity of Syria.”

Kurdish politicians have said the border force is required to protect from any threats emanating from Damascus or Ankara.

The US-led coalition declined to comment on Mr Erdogan's threats in a statement.

“Turkey is a valued member of a 74-member Coalition and a Nato partner, sharing our mission to ensure the lasting defeat of [Isis] in Iraq and Syria. It would be inappropriate for us to comment on Mr. Erdogan's remarks,”