Two main ethnic Karen rebel groups that were once rivals along Myanmar's border with Thailand have joined forces to defend themselves against Myanmar's army.

The union of the Democratic Karen Buddhist and Karen National Liberation militias comes days after clashes between them and troops. Clashes erupted after the rebels resisted government efforts to recruit them to be a part of a border guard force.

The groups are confident that the newfound solidarity will strengthen them against Myanmar's military, who they say are planning to launch an all-out offensive on them soon.

Aela Callan reports from the Myanmar-Thailand border, where she met Karen rebel leaders.