The phone conversation followed a recent U.S. decision to form a border protection army in Syria with the terrorist PYD/PKK.
Turkey’s Recep Tayyip Erdogan told Jens Stoltenberg, NATO’s secretary general, that in the face of recent developments in Syria, Turkey will take all necessary measures to protect its national security.
Erdogan said “the creation of a force” expected to be led by the SDF command and controlled by the PYD/YPG terrorist organization is entirely unacceptable to Turkey.
Stressing that Turkey -- a 65-year member of the alliance -- is an important country for NATO, Stoltenberg said that NATO was not consulted on the matter of creating this so-called army.
Earlier on Tuesday, Erdogan called on NATO to fulfill its responsibilities.
"NATO! You are obliged to take a stand against those who harass the borders of one of your partners," he said.
On Saturday, Turkish security forces hit several PKK/PYD targets in Afrin to prevent a "terror corridor" from forming along Turkey's borders.
An Afrin operation would follow Turkey's successful seven-month Operation Euphrates Shield, which ended in March 2017.
The U.S.-led Syria coalition issued a statement on Sunday saying that it was working with the SDF to set up and train a Syrian border protection force.
Turkey has long protested U.S. support for the PYD/PKK, the Syrian offshoot of the PKK terrorist group, while Washington has brushed off Turkey’s concerns, calling the PYD/PKK a "reliable ally" in its fight against Daesh in Syria.
Listed as a terrorist organization by Turkey, the U.S. and the EU, the PKK has waged a terror campaign against Turkey for more than 30 years.