ANKARA, Turkey -- Turkey blamed Kurdish militant groups at home and in neighbouring Syria on Thursday for a deadly suicide bombing in Ankara and vowed strong retaliation for the attack -- a development that threatens to further complicate the Syria conflict.

The rush hour car-bomb attack on Wednesday evening targeted buses carrying military personnel, killing 28 people and injuring dozens. It came as Turkey is grappling with an array of serious issues, including renewed fighting with Kurdish rebels, threats from Islamic State militants and the Syria refugee crisis. The attack was the second deadly bombing in Ankara in four months.

Prime Minister Ahmet Davutoglu told reporters that a Syrian national with links to Syrian Kurdish militias carried out the attack in collaboration with Turkey's own outlawed Kurdish rebel group, the Kurdistan Workers' Party, or PKK. Davutoglu also blamed Syria's government for allegedly backing the Syrian Kurdish militia.

The attack came as Turkey had been pressing the U.S. to cut off its support to the Kurdish Syrian militias that Turkey regards as terrorists because of their affiliation with the PKK. The U.S. already lists the PKK as a terror group. But Washington relies heavily on the Syrian Democratic Union Party, or PYD, and its military wing, the People's Protection Units, or YPG, in the battle against the Islamic State group and has rejected Turkish pressure.

An Arab-Kurdish alliance dominated by the YPG has made significant advances against IS and other insurgents near the Turkish border in the past week. On Wednesday, the U.S.-backed group known as the Syria Democratic Forces launched an offensive to try to reach Shaddadeh, a major Islamic State group stronghold in Syria's northeastern Hassakeh province, which borders Iraq.

Turkish artillery has been shelling PYD and YPG positions along its border in Syria, apparently concerned by recent gains by the militias in the area. Any Turkish escalation against the PYD is likely to further strain ties with the U.S.

"It has been determined with certainty that this attack was carried out by members of the separatist terror organization together with a member of the YPG who infiltrated from Syria," Davutoglu said, identifying the bomber as a Syrian man named Salih Neccar who was born in 1992.

Neccar was born in the mostly Kurdish Syrian town of Amouda, near the Turkish border, according to Davutoglu.

At least 14 people have been arrested since Wednesday in connection with the Ankara attack, President Recep Tayyip Erdogan said, adding that the number of detained suspects is likely to increase.

The leader of the main Syrian Kurdish group, Salih Muslim, denied that his group was behind the Ankara attack in an interview with The Associated Press and warned Turkey against taking ground action in Syria.

Cemil Bayik, the leader of a Kurdish umbrella organization that includes the PKK, said in an interview with the pro-Kurdish Firat News agency that he did not know who was behind the attack in Ankara. But he suggested that Kurdish militants, angered by Turkish military operations in the country's southeast, may have acted independently.

Erdogan insisted that evidence obtained by Turkish authorities pointed to the Syrian Kurdish group.

"Despite the fact that their leader says they have nothing to do with this, the information and documents obtained by our Interior Ministry and all our intelligence organizations shows that (the attack) was theirs," Erdogan said.

Erdogan said the attack would show the international community the strong links that exist between the PKK and the Syrian Kurdish militias.

"Those who directly or indirectly back an organization that is the enemy of Turkey risk losing the title of being a friend of Turkey," Davutoglu said in an apparent reference to Washington. "It is out of the question for us to excuse a terror organization that threatens the capital of our country."

Paul T. Levin, head of the Stockholm University Institute for Turkish Studies, told the AP: "it will be interesting to see how the United States reacts because they view the YPG as an ally ... Will they (the United States) shift their view on YPG?"

The Britain-based Syrian Observatory for Human Rights, an opposition activist group that monitors the Syria war, said that U.S.-backed Syria Democratic Forces fighters had seized eight villages around the town of Hol in the past 24 hours.

On Thursday, the ambassadors of the five permanent UN Security Council member states were invited separately to Turkey's Foreign Ministry and were being briefed on the Ankara attack, a ministry official said. The ambassadors of Germany and the Netherlands as well as the head of the European Union delegation to Turkey were also invited. The official spoke on condition of anonymity because she wasn't authorized to speak publicly on the issue.

The five permanent members are Britain, China, France, Russia and the U.S.

Hundreds of people have been killed in Turkey in renewed fighting following the collapse of the peace process between the government and the Kurds in July. The fighting has displaced tens of thousands in the southeast as Turkey carried out large-scale military operations against PKK-linked militants.

On Thursday, six soldiers were killed in southeastern Turkey after PKK rebels detonated a bomb on a road linking the cities of Diyarbakir and Bingol, the state-run Anadolu Agency reported.

Turkey has also been helping efforts led by the U.S. to combat the Islamic State group in neighbouring Syria, and has faced several deadly bombings in the last year that were blamed on IS.

The Syrian war itself is raging along Turkey's southern border. Recent airstrikes by Russian and Syrian forces have prompted tens of thousands of Syrian refugees to flee to Turkey's border.

Turkey's military, meanwhile, said its jets conducted cross-border raids against Kurdish rebel positions in northern Iraq, hours after the Ankara attack, striking about 60-70 PKK rebels. The Turkish jets attacked PKK positions in northern Iraq's Haftanin region, hitting the rebels, which it said included a number of senior PKK leaders. The claim couldn't be verified.

In October, suicide bombings blamed on IS targeted a peace rally outside the main train station in Ankara, killing 102 people in Turkey's deadliest attack in years.

Bassem Mroue in Beirut, and Jan M. Olsen in Copenhagen, Denmark, contributed to this report