Turkey announced it will oppose NATO’s plans for enhancing defence of three Baltic countries if the Alliance does not recognize militant groups that Ankara defines as terrorist, President Tayyip Erdogan said on Tuesday, ahead of a NATO alliance summit in London.

Relations between Turkey and its NATO allies have been strained over a number of issues, ranging from Ankara’s decision to purchase Russian air defense systems S-400  to policy vis-à-vis Syria. Several NATO members condemned Turkey’s decision to launch an offensive into northeastern Syria against the Kurdish People’s Protection Units (YPG) militia.

Ankara has refused to back NATO defense plan for the Baltics and Poland until it receives more support for its battle with the YPG, which it assesses as a terrorist organization.