Turkey has ramped up its rhetoric to threaten an imminent cross-border incursion against Kurdish militia in Syria but the attitude of Russia and to a lesser extent the United States will determine the nature of the operation, analysts say.

The Kurdish People's Protection Units (YPG) militia controls key northern Syrian towns including Manbij and Afrin, and is an ally of the US but Ankara accuses the group of being a terror organisation.

Tensions have risen to a new peak in the last days after the United States announced plans for a new 30,000-strong border security force in northern Syria that would be composed partly of YPG fighters.

President Recep Tayyip Erdogan threatened to destroy the force, describing it as an "army of terror".

"The preparations have been completed, the operation could start at any moment," Erdogan said this week, as the Turkish army sent dozens of military vehicles and hundreds of additional personnel to the border area.

- 'Needs Russian green light' -

Yet executing the operation on the ground -- especially against a well-populated urban centre such as Afrin -- could prove much harder than making threats in fiery language.

Crucial will be the attitude of Russia, which has worked increasingly closely with Turkey on Syria in the last year but has a military presence in the area where it cooperates with the YPG.

"Can Ankara dare to attack Afrin without getting a green light from Russia? It's a sure 'no' for me," said Metin Gurcan, security analyst at Istanbul Policy Center and Al Monitor columnist.

He said that despite the increasingly inflammatory language from Erdogan, a full operation would require that Russia open Afrin's air space to Turkey and withdraw its soldiers from the area.

Tensions between Moscow and Ankara have grown in the last days as Russia seeks wide attendance at a peace conference on Syria at the end of the month. But Turkey insists it will not attend if the YPG is there.

In a potentially decisive meeting, Turkey's army chief General Hulusi Akar and spy supremo Hakan Fidan held talks in Moscow on Thursday with Russian counterparts on Syria.

- 'Hard to back down' -

"The only external power that can stop an invasion at this point is Russia," said Aaron Stein, resident senior fellow at the Atlantic Council's Rafik Hariri Center.

He said Erdogan had threatened incursions inside Syria "once a week, every week" for the past year since the Euphrates Shield incursion Turkey launched in August 2016, which ended the following spring.

"What makes this different is that the rhetoric is far more specific, pointed and hostile towards the US. I assume that he will carry out his threat, but the scale of the operation is still an unanswered question," he said.

Turkish Foreign Minister Mevlut Cavusoglu insisted Thursday that Russia would not oppose an Afrin operation, saying that Ankara needed to coordinate with Moscow to ensure its military observers on the ground were not harmed.

Aaron Lund, a fellow with The Century Foundation, said that "it would be hard for Erdogan to back down at this point" following such "loud and persistent" threats.

He said if the operation turned into full-out combat, much of the actual fighting would be done by Turkey-backed Syrian rebel forces like in the Euphrates Shield operation.

But he added that Afrin has tough terrain and was well fortified while the "YPG is a disciplined and effective force."

- 'Not a US problem' -

Moreover, any Turkish intervention may not find the warmest of receptions in Washington, which has closely cooperated with the YPG as its main ally on the ground in the fight against the Islamic State extremist group.

Yet Afrin -- which lies to the west of the main Kurdish zone of influence in Syria -- may not be a prime concern of Washington which is more interested in the Kurdish-controlled areas stretching east to the Iraqi border.

"As far as I can tell, the Americans do not view Afrin as being their problem," said Lund, saying the American military was in Syria on a "fairly narrow counter-terrorism mandate."

"That said, they must be worried that this could create trouble for them" especially if Turkey fired on YPG-controlled areas to the east with a US presence, he said.

Stein said there was a "recognition in Washington that this is a Turkish show" and "little to be done to dissuade Erdogan" if he chooses to go ahead with the incursion.