Erdogan Says 'No Doubt' Syrian Kurd Fighters behind Ankara Attack

President Recep Tayyip Erdogan said on Friday that Turkey had "no doubt" that Syrian Kurdish fighters were behind an attack on a convoy of military buses in Ankara that left 28 people dead.

"We have no doubt that the perpetrators are the YPG and PYD," Erdogan said in Istanbul, referring to the main Syrian Kurdish militia and their political wing. 

Ankara has insisted that the Syrian Kurdish People's Protection Units (YPG) and Democratic Union Party (PYD) were behind the attack, although its claims have met with skepticism from chief NATO ally the United States.

Wednesday's attack -- blamed on a Syrian suicide car bomber -- struck at the heart of Ankara an an area where institutions including the army headquarters and parliament are concentrated. 

Ankara prosecutors said Friday that three more suspects had been detained in the investigation, bringing the total to 17, and that their links to Kurdish militants were being investigated.

"Who was the suicide bomber? Of course he was from the YPG," said Erdogan. He said "three names" were being investigated for a particularly active role in the bombing, without giving further details.

The attack risks further straining ties between Turkey and the United States, which works with the YPG as an effective force in the fight against Islamic State (IS) jihadists in Syria.

- ' Convince our friends' -

Erdogan said Turkey was "saddened" by the stubbornness of the West in not linking the YPG to the Kurdistan Workers' Party (PKK) which has waged a three-decade insurgency against the Turkish state and is recognized as a terror group by the United States and EU.

He added he would speak to U.S. President Barack Obama by phone later Friday to warn him over "the weapons support they (the United States) give to those organizations," referring to the PYD and YPG.

"This incident will help our friends -- who have so far failed to be convinced -- better understand how strong the links are between the YPG and PYD in Syria and the PKK in Turkey," he said.

U.S. State Department spokesman John Kirby said Thursday it was still an "open question" who had carried out the Ankara attack.

"We're in no position to confirm or deny the assertions made by the Turkish government with respect to responsibility."

But Erdogan refused to be drawn into a debate with Kirby, who has angered Turkey by repeatedly defending U.S. support of the YPG in the last days.

"He (Kirby) is not my interlocutor. I am going to speak to Obama at five (1500 GMT)," Erdogan said.

In Ankara meanwhile, eight victims of the attack were laid to rest following a funeral ceremony at the city's vast Kocatepe Mosque attended by Prime Minister Ahmet Davutoglu and chief of staff General Hulusi Akar.

Six of those buried were civilian employees of the military and two were soldiers. In all 20 soldiers of varying ranks were killed in the attack, seven civilian employees and a young female journalist.