ISTANBUL  - President Recep Tayyip Erdogan on Friday warned the United States against backing Kurdish fighters in Syria, saying there was "no doubt" they were behind a deadly attack on the Turkish military in Ankara.

The attack on a convoy of military buses in Ankara left 28 people dead and has further complicated the search for an end to the Syrian conflict by creating a new bone of contention between Turkey and its chief NATO ally the United States

"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 but although its claims have met with scepticism from Washington. 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 recognised as a terror group by the United States and EU.

He added he would speak to US President Barack Obama by phone later Friday to warn him over "the weapons support they (the United States) give to those organisations," 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. "Who was the suicide bomber? Of course he was from the YPG," said Erdogan.

The United States works with the YPG as the sole truly effective force on the ground in the fight against Islamic State (IS) jihadists in Syria and has shown no sign of changing its stance on the group.

US 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 US 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.

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 six more suspects had been detained in the investigation, bringing the total to 20, and that their links to Kurdish militants were being investigated.

Erdogan said "three names" were being investigated for a particularly active role in the bombing, without giving further details. 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.