About 10 attackers opened fire and threw pipe bombs at soldiers at a security check point near a residential area in Narathiwat province Wednesday evening before fleeing, according to the regional Internal Security Operations Command on Thursday.
It said six civilians were injured during the attack, including a 10-year-old boy who remains hospitalised in stable condition.
Photos released by authorities showed a group identified as the attackers, dressed in black clothing and hats and riding on the back of a black ute.
Security officials said the truck was later found abandoned and set on fire about 20km away from the scene.
They said the attackers had stolen three AK-47 rifles, three bulletproof vests and three mobile phones from the soldiers.
Narathiwat, along with Yala and Pattani, are Thailand's southernmost provinces and are the only ones with Muslim majorities in the predominantly Buddhist nation.
Since 2004, the provinces have been the site of a Muslim separatist insurgency that killed thousands.
Muslim residents have long complained of being treated like second-class citizens in Thailand, and separatist movements have been periodically active for decades.
Heavy-handed crackdowns have fuelled the discontent.
Fighting continues to this day, but at a lower level.
The Internal Security Operations Command Region 4, which oversees security operations in southern Thailand, vowed in a Facebook post to "expedite the investigation and search for the perpetrators with the best efforts".
Prime Minister Anutin Charnvirakul condemned the attack, saying it was a "terrorist attack that clearly aimed at killing government officials, and a serious threat to national security", according to government spokesperson Rachada Dhnadirek.
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