Ankara (AFP) - Turkish President Recep Tayyip Erdogan on Wednesday said Syrian Kurdish militia forces must remain outside the scope of a ceasefire agreed between Syria's warring parties, in a blow to the deal days before it is due to be enforced.

Lashing out at Western policy in Syria, he said the Syrian Kurdish Democratic Union Party (PYD) and its People's Protection Units (YPG) militia were a "terror group" just like Islamic State (IS) jihadists and the Al-Qaeda affiliated Al-Nusra Front.

The issue has already provoked a rare rift between Ankara and its chief NATO ally Washington, which works closely with the YPG as an effective fighting force against IS in Syria.

"If Daesh (IS) and Al-Nusra are kept outside the ceasefire, then the PYD-YPG must similarly be excluded from the ceasefire for it is a terrorist group just as they are," Erdogan told local officials in Ankara.

The United States and Russia-brokered ceasefire calls for a "cessation of hostilities" between forces loyal to President Bashar al-Assad and opposition groups, and is due to take effect at midnight on Friday Damascus time.

But the truce agreement, clinched Monday, does not apply to jihadists such as the Islamic State group and Al-Nusra Front or other groups deemed terror outfits by the UN.

Erdogan said Turkey welcomed the Syria ceasefire as "positive in principle" and said: "We support a ceasefire that will help our Syrian brothers breathe."

- 'Matter of survival' -

But he warned that Turkey would be "on alert" against any steps that threaten its national security and would continue to closely monitor the ceasefire process.

Turkey says the YPG is a branch of the outlawed Kurdistan Workers' Party (PKK), which which has waged a three-decade insurgency against the Turkish state and is recognised as a terror group by the United States and EU.

Ankara blames the YPG for last week's suicide car bomb attack in the Turkish capital that killed 29 people and fears the creation of a Kurdish stronghold along its southern border.

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A militant Kurdish group claimed the suicide car bombing of a convoy of military buses in Ankara but Turkish officials said the bomber was a Syrian Kurdish national working on behalf of the PYD.

The PYD has denied any involvement in the attack.

On successive days last week, Turkish artillery shelled YPG targets inside Syria, defying US calls to cease firing.

The president said its key ally Washington's approach towards the PYD did not bode well with the "alliance spirit."

"This is a matter of survival for Turkey," Erdogan said. "Believe me, I barely understand how clearly we can explain for our allies to understand that they are at a crossroads on this issue."

He quipped that if fighting IS was the only criteria, then the West should cooperate the Al-Nusra front which often battles the even more extremist IS jihadists.

Erdogan said the PYD and the YPG must be designated as "a terrorist organisation" by Turkey's allies

"To accept it as a terrorist organisation, will it require that the PKK and PYD explode their bombs not in Ankara but in other (foreign) capitals?" he said.