American Secretary of State Hillary Clinton has expressed concern at the increased presence of Syrian troops along the border with Turkey.
Hundreds of refugees have broken through barbed wire and crossed into Turkey after Syrian tanks and snipers reportedly entered the village of Khirbet al-Jouz.
Mrs Clinton has described the troop movements are "very worrisome", the BBC reports.
She says it could increase the risk of a border clash with Turkey and worsen the plight of refugees.
More than 1300 people are said to have been killed since a crackdown on anti-government protests began in March.