The Burmese military dismissed independent and social media reports claiming the deceased were local villagers who had been abducted by the Burmese army.
Bodies were exhumed from shallow graves in a cornfield, nearly a week after they were detained by a Burmese army unit on suspicion of being rebel supporters.
The 13, detained four months ago in the Arakan State capital, apparently confessed to having connections with the AA.
The military is refusing to cooperate in the investigation into the rape and murder of two Kachin teachers in northern Shan State.
Love them or loathe them, the Burmese the military has run Burma for more than 50 years, far longer than most regimes last.
The Burmese army is continuing its intervention into an ongoing conflict between two ethnic rebel groups in Shan State, according to government media.