The Turkish government has blamed two Kurdish groups for a bomb attack in the country’s capital that killed 28 people, including one the United States considers an ally in the fight against the Islamic State.

Turkish Prime Minister Ahmet Davutoğlu said Wednesday the Turkey-based Kurdistan Workers Party, or PKK, worked with a wing of the Syria-based Democratic Union Party, or PYD, to carry out Wednesday’s attack.

At least 28 people were killed and 61 were injured when a car bomb detonated near military buses stopped at traffic lights in Ankara’s administrative hub. At least 20 of those killed were military personnel. Turkish leaders said they have identified the perpetrator of the attack as a Syrian national. The PYD and YPG denied any involvement in the attack.

Davutoğlu criticized the U.S. for supporting Kurdish militias in the mission against the Islamic State. The U.S. backs the PYD’s armed militia, the People’s Defense Units, or YPG, which is battling the Islamic State in Syria.

“We can’t excuse any NATO ally, including the U.S.” of having “links with a terrorist organization that strikes us in the heart of Turkey,” Davutoğlu said in a televised speech Wednesday.

Turkey considers the PYD and YPG affiliates of the PKK, which the government considers a terrorist organization. The Turkish government and the PKK, the Kurdish separatist group, have been locked in a decades-long armed conflict in the country’s southeast, punctuated only by a handful of failed cease-fires.

Davutoğlu’s comments reflect increasingly strained relations between Washington and Ankara, two longtime allies. Both the U.S. and Turkey believe the resolution to Syria’s five-year-long civil war lies in removing the country’s president, Bashar al-Assad, from power. Both sides arm and train Syrian rebels against Assad’s government. And both are part of the broad coalition fighting the Islamic State. But while the U.S., like Turkey, lists the PKK as a terrorist organization, it has dismissed Turkey’s requests that the U.S. cut off its support from the YPG, calling the group “an effective fighting force.”

To make matters worse, in recent weeks, U.S.-backed YPG fighters have seized territory from Syrian rebels who are facing a major Syrian-army offensive, supported by Russian airstrikes, near Aleppo, just south of the Turkish border. Turkey, fearful such action might stoke separatist sentiment among Turkish Kurds, has responded by firing artillery shells at the militia across the border. (In an interesting twist, Syria’s representative to the United Nations said this week, after Turkey began shelling PYD positions in northern Syria, that the Syrian government supports PYD, too.)

The U.S. has called on the YPG and the Turkish military to stop these military operations in northern Syria, a move that has angered Turkish President Recep Tayyip Erdoğan.

“At the moment, I have difficulty in understanding America, which still hasn’t called or still cannot call the PYD and the YPG as terrorists and which says, ‘Our support for the YPG will continue,’” Erdogan said Wednesday.

Turkey vowed retaliation for the Ankara attack, launching airstrikes against PKK bases in northern Iraq, where Kurds have a stronghold in that country’s autonomous Kurdish region, hours after the Ankara explosion. Turkish cities have repeatedly been the targets of terrorist attacks in recent months, but most have been attributed to the Islamic State, which controls large parts of Iraq and Syria. Washington and Ankara have been split on the use of  Syrian Kurdish militias against the Islamic State since the terrorist group emerged as a global threat in 2014. They have publicly maintained an image of harmony—administrations working toward the same end—but this week’s attack, coupled with intensified fighting along the Turkey-Syria border, threatens to further strain their working relationship.