Kurdish militias are closing in on an important Islamic State-held town in Syria near the Turkish border.
Retaking Tal Abyad could cut off a transit and supply route for Islamic State fighters.
Thousands of Syrian civilians have fled the area, anticipating fierce fighting between the Kurds and the militants.
Syrians managed to cross into Turkey Sunday after some Turkish soldiers kept them from entering. One reporter said several soldiers let the refugees cross while another journalist said he saw people cutting through a fence.
Fighting has driven more than a million Syrian refugees into Turkey, and Turkish President Recep Tayyip Erdogan is worried the refugee crisis could worsen.
He also said Sunday he is concerned that two Kurdish groups - the Democratic Union Party and the outlawed Kurdistan Workers Party - could gain ground along Turkey's border with Syria.
Kurds have long fought a guerilla war against Turkey for an independent Kurdish state. Erdogan believes U.S.-led coalition airstrikes along just inside Syria could carve out territory for the Kurds.
Also in Syria Sunday, car bombs wounded 27 people in two government-controlled neighborhoods in the city of Homs. No one has claimed responsibility.
Homs is one of the cities where the uprising against Syrian President Bashar al-Assad began four years ago and has been a frequent target of bombings.