Esper, in particular, reminded his Turkish counterpart of the need to ensure the safety of Syrian residents who find themselves in territory controlled by the Turkish army.
He also confirmed that the United States will not participate in patrolling the security zone in Syria and will continue to withdraw its troops.
Earlier, US and Turkish leaders Donald Trump and Recep Tayyip Erdogan held talks on the situation in Syria.
On October 17, Ankara and Washington agreed to suspend Operation Source of Peace in northeastern Syria to allow Kurdish forces to leave the security zone.