Turkish Prime Minister Ahmet Davutoglu, right, walks with Hulusi Akar, chief of the General Staff of the Turkish armed forces, during a condolence visit at the General Staff headquarters in Ankara on Feb. 18, 2016, the day after a bombing killed 28 in the capital. (Hakan Goktepe/Turkish prime minister’s press office via AFP/Getty Images)

Turkey blamed Syrian Kurds on Thursday for a suicide bombing that killed 28 people in the capital, Ankara, and vowed to retaliate, threatening new complications for the war in neighboring Syria and for the U.S. fight against the Islamic State.

The bombing coincided with heightened tensions between the Syrian Kurdish People’s Protection Units (YPG) and Turkey, which has fired artillery into Syria in recent days to prevent Kurdish advances toward the Turkish border. The allegation that the YPG was involved in the bombing Wednesday night of a bus carrying Turkish military personnel raised the specter of deepening involvement by Turkey in the war in Syria. Of the fatalities, 27 were Turkish service members.

The attack also served to highlight growing fissures between Turkey and the United States over U.S. support for the YPG in the fight against the Islamic State. Washington in recent days has strenuously rejected Turkish efforts to force it to renounce the YPG, which Ankara calls a terrorist organization.

As Turkey resumed the artillery strikes late Thursday, State Department spokesman John Kirby said the United States still had not determined responsibility for the bombing. “As far as we’re concerned, that’s an open question,” he said.

“Clearly it is an act of terrorism,” he added, urging both sides to show restraint and to focus on the fight against the Islamic State.

“Some of the strongest fighters against Daesh inside Syria have been Kurdish fighters,” Kirby said, using the Arabic acronym for the Islamic State. “The side that we all need to be on here is the counter-Daesh side.”

Prime Minister Ahmet Davutoglu earlier said a member of the YPG carried out the bombing in collaboration with Turkey’s Kurdistan Workers’ Party, or PKK, which has been waging a decades-long war for autonomy on behalf of Kurds in Turkey. He also accused Syrian President Bashar al-Assad of complicity, citing claims in the past by Assad and members of his government that they supply the YPG with arms.

Authorities named the bomber as Saleh Najjar from the Kurdish town of Amudah in northern Syria and said 14 people have been detained in connection with the attack.

“A direct link between the attack and the YPG has been established,” Davutoglu told reporters in Ankara. “The attack was carried out by the PKK together with a person who sneaked into Turkey from Syria.”

The YPG swiftly denied any link to the bombing. The group said the Turkish government was accusing the YPG to justify further attacks against the rapidly expanding Kurdish enclave known as Rojava that the Kurds are carving out in northern Syria. Turkey has vowed to prevent the creation of an autonomous Kurdish entity along the Syrian border, because of fears that it would encourage Turkish Kurds to seek their own state.

The allegation is “part of an attempt by the Turkish prime minister to establish new foundations for their attacks on Rojava during the Syrian crisis,” the YPG said in a statement.

“We say to the people of Turkey and the international community: there is no relation between us, the YPG, and yesterday’s incidents in Ankara,” it added.

However, Turkish President Recep Tayyip Erdogan said the bombing proved Turkish assertions that the YPG is an affiliate of the PKK, which the United States and Turkey have designated as a terrorist organization.

“With regard to our allies in the international arena, it will be understood how strong the [Kurdish Democratic Union Party] and YPG’s connection with the PKK in Turkey is,” he said. “This will make it possible for our allies to understand us better.” The YPG is the military wing of the Democratic Union Party.

The Ankara bombing played directly into the vast complexities of the fight underway just beyond Turkey’s borders in the northern Syrian province of Aleppo, and it seemed calculated — by whoever carried it out — to provoke a Turkish response.

Recent advances by the YPG in the northern countryside of the province have come at the expense of Syrian rebels — backed by Turkey and, in some instances, the United States — who have been fighting an increasingly desperate battle for survival in the face of punishing Russian airstrikes. Government loyalist forces, chiefly comprising Iranian-backed Shiite militias from Lebanon, have seized territory closer to the city of Aleppo, while the YPG has pushed into areas closer to the Turkish border.

The Kurds are now threatening to seize the strategically vital Syrian border town of Azaz, which Erdogan earlier this week warned would be a “red line” for Turkey. Russian Foreign Ministry spokeswoman Maria Zakharova warned Turkey against sending troops to Syria, saying that “any invasion into the territory of a sovereign state is illegal.”

The United States has appealed in the past week to Turkey to stop its shelling of the advancing Kurdish forces and to the YPG to halt its advances, without effect.

Turkey’s options are constrained by the strong presence of the Russian air force in the area and by the restrictions of Turkish membership in the NATO alliance. Russia has repeatedly warned Turkey that it intends to exact vengeance for the downing of a Russian plane that strayed into Turkish airspace in November. Turkey’s NATO allies are obliged to come to its defense only if it comes under attack, not if it embarks on unilateral action.

Wednesday’s bombing in Ankara also coincided with a massive Turkish military crackdown against Kurdish communities in the southeast of the country that has killed hundreds, displaced hundreds of thousands and turned cities and towns into war zones.

Cemil Bayik, a PKK leader who lives in northern Iraq, said he did not know who had carried out the bombing. But he told a Turkish news agency that it may have been “an act of retaliation for the massacres in Kurdistan,” a reference to the brutal military campaign.

The PKK has frequently targeted military convoys and off-duty military personnel, and its attacks have intensified as the Turkish military has stepped up its onslaught against the Kurds. A bombing against a military vehicle on Thursday in southeastern Diyarbakir province killed six people, according to the Turkish military.

Turkish warplanes carried out fresh raids overnight against PKK bases in northern Iraq, a frequent retaliatory target of Turkish airstrikes.

Michael Birnbaum in Moscow and Karen DeYoung in Washington contributed to this report.

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