State media said Russian-backed Syrian forces entered the town of Manbij as part of a deal reached with Kurdish-led forces previously allied with the US.
Meanwhile, Turkish troops and allied Syrian militias were gathering near the town as they continued their incursion.
Turkey's offensive aims to push Kurdish forces from the border region.
Syrian state media said government forces had entered Manbij, in the area where Turkey wants to create a "safe zone" cleared of Kurdish fighters. Earlier, the army pushed into Tal Tamer and Ain Issa, where residents celebrated their arrival.
The deal is seen as a boost for President Bashar al-Assad as his troops return to those areas for the first time since 2012. The deployment came hours after the US announced that up to 1,000 American troops would leave northern Syria.