Turkey has sent military reinforcements along its border with Syria, according to state media, as Ankara, Damascus and Moscow all attacked a US plan to form a new border security force in the war-torn country's northeast.

Citing military sources, Anadolu Agency reported on Monday that two dozen armoured vehicles had entered Reyhanli district of southeastern Hatay province.

A separate 20-vehicle army convoy, which included tanks, had also arrived in Viransehir district of southeastern Sanliurfa province.

The forces were sent to assist the military units already deployed along the Syrian border, said Anadolu.

The deployments came a day after Turkish President Recep Tayyip Erdogan said that a military operation in northern Syria against the city of Afrin - controlled by the Syrian Kurdish People's Protection Units (YPG) - would be launched "in the days ahead".

A senior Syrian Kurdish official said on Sunday that fighting between the YPG and Turkish forces was already under way, while Anadolu reported on Monday that intense smoke was coming out of Afrin's Nesreyieh region.

'Drown this terror army'

Separately on Monday, Erdogan said the US was working to form a "terror army" on his country's southern border by training a new force in Syria that includes Kurdish fighters.

"What we are supposed to do is to drown this terror army before it comes into being," he said in an address in the capital, Ankara, calling the Kurdish fighters "back-stabbers" who will point their weapons to the US in the future.

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His comments came after reports revealed Washington's plan to establish a 30,000-strong new border security force with the involvement of Kurdish fighters in northern Syria.

According to media reports quoting US officials, the US-led coalition fighting the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) in Syria says the force will secure areas along Syria's border to the north with Turkey and to the east with Iraq.

At least half of it will be made up of the Syrian Democratic Forces (SDF), an umbrella group of fighters dominated by the YPG, seen by the US a highly effective ground force against ISIL.

But the YPG is considered by Turkey to be a "terrorist group" with ties to the banned Kurdistan Workers' Party (PKK), which has waged a decades-long fight inside the country.

PKK is blacklisted as a terrorist organisation by Turkey and its Western allies. More than 40,000 people in Turkey have been killed since the 1980s after the PKK launched its armed campaign.

Erdogan said Turkey's armed forces had completed preparations for an operation against Afrin and the town of Manbij.

Warning Turkey's allies against helping "terrorists" in Syria, he said: "We won't be responsible for consequences".

Syria, Russia react

Later on Monday, an official source in Syria's foreign ministry denounced the US plan for the formation of the border force.

"Syria strongly condemns the US announcement on the creation of militias in the country's northeast, which represents a blatant attack on the sovereignty and territorial integrity and unity of Syria, and a flagrant violation of international law," said the source, according to state news agency SANA.

"Syria considers any Syrian who participates in these militias sponsored by the Americans as a traitor to their people and nation, and will deal with them on this basis."

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Russia also attacked the US plan, calling it a plot to partition Syria.

"The [US'] actions that we have been observing indicate that the US does not want to keep Syria as a state in its current borders," Russian Foreign Minister Sergey Lavrov said in a press conference in Moscow.

"The US wants to help the Syrian Democratic Forces to set up some border security zones," he added.

"What it would mean is that vast swaths of territory along the border of Turkey and Iraq would be isolated, it's to the east of the Euphrates river. There are difficult relations between Kurds and Arabs there. If you say that this zone will be controlled by the forces supported by the US, there will be a force of 30,000 people."

Lavrov said that the development would be "a very big deal", raising "a lot of question marks".

"There is a fear that they are pursuing a policy to cut Syria into several pieces," he added.

US arming of YPG

US President Donald Trump decided to arm YPG fighters, despite Turkey's objections and a direct appeal from Erdogan at a White House meeting in May 2017.

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The US arms shipments began before the launch of a months-long offensive to remove ISIL from the Syrian city of Raqqa, its self-declared capital. The YPG played a prominent role in the eventual defeat of the group later in 2017.

Tensions between US and Turkey - two NATO allies - remain high, despite Trump saying last November that Washington would no longer supply weapons to the YPG.

On Sunday, Hediye Yusuf, a senior Syrian Kurdish official, called Turkey's operation against Afrin a "violation" that "undermines international efforts to reach a political solution in Syria".

Turkey has been workxing closely with Russia and Iran to end the long-running Syrian war, despite Moscow and Tehran supporting Syrian President Bashar al-Assad - and Ankara backing the anti-Assad opposition.

In 2016, Turkey began a military campaign called Euphrates Shield Operation, which targeted ISIL and the YPG.

That eight-month operation officially ended in March 2017.