Myanmar's army said it shot dead six detainees in a village school Thursday while holding 275 men on suspicion of being Arakan Army (AA).
Security forces are battling the AA, a group pushing for more autonomy for the state.
More than 30,000 people have been forced from their homes in clashes with the AA since January.
On Tuesday, troops descended on Kyauk Tan village in Rathedaung township, separating men aged between 15 and 50, local and military sources confirmed.
Army spokesman Brigadier General Zaw Min Tun told AFP Thursday that 275 people "were being temporarily held in the village school for investigation".
Shots were fired in the school early Thursday morning when the detainees tried to "attack the security forces" who had no option but to shoot, Zaw Min Tun said.
"Six were killed, eight were injured and four escaped," he said, adding warning shots had been first been fired and the wounded had been sent to hospital.
The north of Rakhine state is in strict lockdown, making any independent verification difficult.
Rakhine is one of Myanmar's poorest regions and is deeply divided along ethnic and religious lines.