Suspected Muslim militants killed five soldiers and wounded two others in an ambush today, police said.
An army radio message said the militants raided an outpost, wounding five soldiers and escaping with five guns, but police at the scene said five were killed.
"The soldiers were taking an evening break when dozens of militants attacked them, and these troops had no way to defend themselves," Police Colonel Thanongsak Wangsupa, of Narathiwat province, said.
It is the highest number of security personnel killed by suspected rebels in a single attack since the insurgency began in January last year.
The attackers cut trees to block the road and placed spikes to prevent counter-attacks by soldiers, the radio said.
Suspected militants also beheaded a villager in southern Thailand on the eve of the Islamic holy month of Ramadan.
The head and torso of a man, thought to be in his 50s, were recovered beside a road in a rubber plantation in Yala, one of the three southernmost provinces hit by the violence in which more than 900 people have died.
Police said villagers told them about the beheaded man yesterday, but were too scared of booby traps or ambushes to examine the scene until today.
The policeman said it was unclear if the victim was Buddhist or Muslim, although as with previous beheadings of Buddhists, a note was left beside the victim's head saying the murder was in retaliation for police actions.
The decapitation was the 11th in 21 months of violence.
"You have arrested innocent people from the village. I have killed innocent people in return," the policeman quoted the handwritten Thai-language note as saying.