Mr Esper told reporters that, under current plans, about 1,000 soldiers would be redeployed to help stop the resurgence of Islamic State (IS).
President Donald Trump has previously pledged to bring US troops home.
The US withdrawal from northern Syria paved the way for a Turkish military offensive against Kurdish fighters.
Both sides have accused the other of breaching a US-brokered ceasefire.
Ankara views one of the prominent militias in the Kurdish forces as terrorists, and wants to create a “safe zone” buffer inside Syria.