Turkish President Recep Tayyip Erdogan on Monday threatened to destroy a planned US-backed 30,000-strong border security force in northern Syria, describing it as an "army of terror".

Turkey fears that the mooted new force will be comprised of the Kurdish People's Protection Units (YPG) militia, whom Ankara accuses of being terrorists and have been a constant bone of contention in Turkey-US ties.

The new upsurge of tensions between Ankara and Washington also comes as Erdogan has repeatedly threatened that Turkey could launch a cross-border operation to oust the YPG from the Kurdish-held town of Afrin in northern Syria.

"America has acknowledged it is in the process of creating a terror army on our border. What we have to do is nip this terror army in the bud," Erdogan said in a televised speech outside Ankara.

- 'Gang of murderers' -

The YPG played a key role in the Syrian Democratic Forces (SDF) alliance that last year ousted Islamic State jihadists from their stronghold of Raqa in northern Syria.

American officials said at the weekend that the new force was needed as the focus shifted from fighting IS extremists to border security in northern Syria.

Erdogan on Monday reaffirmed that the Turkish army was ready to launch an operation against the YPG in Syria.

"The preparations have been completed, the operation could start at any moment," he said.

State media said that Turkey deployed two dozen armoured vehicles to the Syrian border on Monday.

Turkey accuses the YPG of merely being the Syrian offshoot of the Kurdistan Workers Party (PKK) which has waged a rebellion in the Turkish southeast for more than three decades and is regarded as a terror group by Ankara and its Western allies.

The row is the latest dispute between Turkey and the United States that goes beyond the YPG and Syria to include arguments over a New York court case, detained US consulate staffers and the prosecution of Erdogan's bodyguards in the United States.

"A country -- which we call as our ally -- is insisting on having an army of terror along our border despite our objections, warnings and well-meaning advice."

"You will be disgraced in front of your strategic ally," he warned Washington. "Don't get between us with the gang of murderers."

"Otherwise we will not be responsible for any unwanted incidents that could emerge later," he added.

The US-led coalition said that 230 people are training to be part of the border security force.

The SDF -- which is dominated by Kurdish elements after the capture of Raqa -- now controls territory bordering Turkey to the north and Iraq to the east, and alongside land controlled by Syrian government forces to the west.

- 'Dividing up Syria?' -

Turkey, despite being a prime foe of the Syrian regime, has in the last months worked closely with President Bashar al-Assad's ally Russia to bring peace to Syria.

Russian Foreign Minister Sergei Lavrov, in an annual Moscow press conference marked by strident anti-US rhetoric, also sounded alarm over the US plan.

He said it raised fears that "the course has been set on dividing up Syria" and was out of step with the peace talks aimed at ending the conflict.

"Just like our Turkish partners... we are awaiting detailed explanation," he said.

For once in agreement in Ankara, the Damascus regime lambasted the plan to create the force, saying it would consider its members "traitors".

A foreign ministry source, cited by state news agency SANA, said the move "represents a blatant attack on the sovereignty and territorial integrity and unity of Syria."

Turkey meanwhile is also keeping a close eye on the northwestern Syrian province of Idlib which is almost entirely controlled by anti-government forces dominated by the jihadist Hayat Tahrir al-Sham (HTS).

It has urged Russia and Iran to rein in their Syrian regime ally and stop bombing the area, in a dispute that has led to rare public tensions with Moscow.

Russia is hoping to hold a Syria peace congress in Sochi on January 29-30 with the specific aim of setting up a new constitution for post-war Syria.

But Turkey has made clear it will go nowhere near the event if there is any representation by the YPG there.