ANKARA, Turkey — Turkey on Thursday blamed Kurdish militant groups at home and in neighboring Syria for a deadly suicide bombing in Ankara, and it vowed strong retaliation, threatening 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 of others. It came as Turkey grapples with an array of issues, including renewed fighting with Kurdish rebels, the threat from Islamic State militants and the Syria refugee crisis. It 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 support to the Kurdish Syrian militias, which 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.