More than 6,300 people, the majority of them civilians, have died in over a decade of conflict pitting troops and police against rebels seeking greater autonomy for the Muslim-majority provinces bordering Malaysia.
A 57-year-old soldier from the southern province of Phatthalung opened fire late on Friday at a barracks in the Yaring district of Pattani province, killing two other soldiers, local police chief Montri Kongwatmai told AFP.
"The soldier killed two other soldiers and then surrendered himself," he said. "Our investigation shows he had a personal conflict with them."
Last November the nation's army chief warned that under- pressure troops in the south need better support after a conscript shot dead four officers before turning the gun on himself.
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The estimated 60,000 troops and police blanketing the area face near-daily roadside bomb attacks and ambushes from shadowy rebel groups. They melt away into remote communities broadly opposed to rule by Buddhist-majority Thailand, which colonised the region more than a century ago.
While they are frequently the target of attacks, security forces also stand accused of widespread human rights abuses of the culturally distinct southern population.