Thirteen Myanmar government soldiers were killed and two were missing after a fight with an unidentified armed group near the border opposite Kra Buri district, Ranong, on Friday, a security source said.
The clash occurred as the Myanmar troops were patrolling the border in the area where 92 Thais were arrested in August last year for encroaching on Myanmar territory, the source said.
The identify of the attackers could not be confirmed.
Myanmar had sought cooperation from Thailand to prevent the attackers fleeing across the border.
Army chief Gen Prayuth Chan-ocha had directed a task force in the district to keep a close watch on the border, the source said.