A bomb blast in the heart of the Turkish capital, Ankara, killed 28 people Wednesday, deepening a sense of crisis enveloping Turkey as it grapples with wars on three fronts.

The explosion appeared to have been caused by a car bomb that detonated as a military bus paused at a traffic light in a central neighborhood that houses the nation’s parliament and government headquarters, according to Turkey’s official Anadolu news agency. In addition to the deaths, at least 61 people were injured in the fireball that engulfed the bus and ignited trees in a nearby park at the height of the evening rush hour.

There was no immediate claim of responsibility for the blast, which came amid increasing challenges from the civil war in neighboring Syria, Turkey’s intensifying feud with Kurds and the rising threat posed by the Islamic State.

A suicide bombing that killed 10 German tourists near the landmark Blue Mosque in Istanbul in January, a double suicide attack that claimed more than 100 lives at a peace rally in Ankara in October and another that killed more than 30 Kurds in southern Turkey last summer were all widely blamed on the Islamic State, although no group asserted responsibility. The attacks followed Turkey’s agreement to join a U.S.-led coalition against the Islamic State and allow U.S. warplanes to launch attacks on the militants from Turkish bases.

In this instance, however, Turkish authorities were swift to blame the Kurdistan Workers’ Party, or PKK, the Kurdish nationalist movement that has been waging war against the Turkish state for most of the past 30 years. Turkey and the United States both designate the PKK as a terrorist organization. The Turkish military in recent months has been pursuing a fierce campaign against PKK fighters and sympathizers, turning many of the Kurdish-majority cities in southeastern Turkey into war zones.

President Recep Tayyip Erdogan immediately canceled a state visit to Azerbaijan and vowed retaliation, although he did not specify against whom.

“Our determination to respond in kind to attacks taking place inside and outside our borders is getting stronger with such acts,” he said in a statement. “It must be known that Turkey will not shy away from using its right to self-defense at any time, any place or any occasion.”

Wednesday’s attack also coincided with Turkey’s recent intervention to halt advances in northern Syria by Syrian Kurdish fighters who have taken advantage of Russian airstrikes in the area to expand territory they control along the Turkish border. Over the weekend, Turkish troops began firing artillery at positions of the Syrian Kurdish YPG, or People’s Protection Units, after Kurdish fighters routed rebels backed by Turkey and the West from several key positions near the border.

The battles in northern Syria have created friction between Turkey and the United States, which has supported the YPG because of the role it plays in battling the extremist Islamic State. Turkey, however, regards the YPG as a terrorist organization because of its close ties to the PKK.

With Turkey blaming the PKK for the latest attack, there is a risk that the government in Ankara will wade deeper into the Syrian conflict, said Soner Cagaptay of the Washington Institute for Near East Policy, who thinks the Kurdish militants are the most likely suspects.

The PKK has a history of targeting barracks and buses carrying off-duty military personnel, although it rarely strikes so brazenly at the heart of the Turkish establishment. If the PKK was responsible, “this is a big escalation,” he said. “I expect a pretty severe reaction from Ankara.”

Turkey also may escalate its crackdown against the domestic Kurdish opposition, a campaign that has killed hundreds and displaced nearly 200,000 people, said Henri J. Barkey of the Woodrow Wilson International Center. He thinks the Islamic State is the most likely perpetrator, because the attack “exacerbates tensions between Kurds and the Turkish government, and that is what they want.”

A third possible suspect is the Revolutionary People’s Liberation Party/Front, a far-left group known as the DHKP-C, which has carried out suicide bombings and other terrorist attacks against Turkish government targets in recent years. Turkish news media reported Tuesday that a suspected DHKP-C suicide attacker had been detained in southern Turkey.

Murphy reported from Washington.

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