Turkish President Recep Tayyip Erdogan on Saturday defended his decision to bomb US-backed Kurdish forces in Syria in an offensive that’s inflaming relations with the US.

Turkey intensified airstrikes and artillery shelling of YPG positions in reprisal for a suicide-bomb attack in the Turkish capital, claiming to have killed at least 58 militants in airstrikes on Friday, after the US shot down a Turkish drone that came too close to US troops in the area on Thursday. The death toll for Kurdish forces, announced by Turkish Defense Ministry, can’t be independently verified.

“We will resolutely continue our fight against terrorism,” Erdogan told his party’s convention in Ankara. “We will hold bloody organizations” to account.

Erdogan spoke a day after Turkey called on the US to stop working with Kurdish YPG militia, who are seen as a critical US ally to defeat Islamic State in Syria but regarded as terrorists by Ankara.

With the two largest armies in NATO, the US and Turkey have good reason to maintain their more than seven-decade alliance. But ties have been strained by the arming and training of the Kurdish YPG forces by the US as well as Turkey’s purchase of a Russian missile-defense system.

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Turkey views the YPG as a terrorist group affiliated with the Kurdish separatist PKK, which the Turkish military has been battling for more than three decades. The PKK is recognized as a terrorist organization by the US and European Union.

Turkey’s military drive is aimed at preventing Kurdish groups who retain control over a large swathe of territory in Syria, which has been mired in a civil war since 2011, from attempting to create a self-rule with support from the US.

By driving away Kurdish forces, Turkey wants to create a buffer zone along its border in the hopes of convincing about 4 million Syrian refugees, the world’s largest refugee population, to return home.