Yildirim held a short news briefing in Istanbul on Sunday afternoon after he visited several victims injured in this morning’s gun attack on a nightclub in the city.
"Terrorism is humanity's common problem. Its aim is to break our unity and brotherhood," Yildirim said, telling reporters Turkey was in a de-facto war with the PKK, the Fetullah Terrorist Organization and Daesh.
Yildirim said a thorough investigation was underway to reveal the identity of the attacker who opened fire on partygoers at the Reina nightclub in Istanbul’s Ortakoy district in the early hours of New Year’s Day.
At least 39 people, including a police officer, died in the assault. Sixty-nine people were also injured.
"Our police and security units are working intensely. We will later share the information gathered with the public," Yildirim said.
Interior Minister Suleyman Soylu earlier told reporters 20 victims had been identified so far.
"Fifteen of the victims are foreigners while the other five are Turkish citizens," the minister said, adding that efforts were continuing to identify the other victims.
Yildirim dismissed claims the attacker had been wearing a Santa Claus costume when he entered the nightclub.
"There are no grounds for these allegations," the prime minister said.
Yildirim also talked with Russian Prime Minister Dmitry Medvedev by phone. The Russian premier expressed his condolences and solidarity with Turkey, according to Turkish prime ministry sources.
Medvedev also assured Yildirim that the two countries would play a constructive role in building peace in the region.