Presidential Communications Director Fahrettin Altun criticized other countries for their support of terrorist groups after the deadly attack in Istanbul's Istiklal Avenue on Sunday.

Terrorist attacks against Turkish civilians are the direct and indirect consequences of the support to terrorist groups, Altun tweeted on Monday.

"They must immediately cease their direct and indirect support if they want Türkiye’s friendship," Altun said, adding that though terrorists think they can intimidate the nation and government through these attacks, it will not work. He went on to add that the Turkish government, under the leadership of President Recep Tayyip Erdoğan, "has never shied away from pursuing and punishing terrorists" wherever they may be.

Authorities earlier said that the initial findings in an investigation into the attack showed that the PKK terrorist group and its Syrian wing YPG might be behind it. The U.S. primarily partnered with the YPG in northeastern Syria under the pretext of fighting against the Daesh terrorist group. Türkiye has strongly opposed the YPG's presence in northern Syria.

Ankara has long objected to the U.S.' support for the YPG, a group that poses a threat to Türkiye and that terrorizes local people, destroying their homes and forcing them to flee.