That blast hit a bus carrying troops to a military base in Pattani province.
Muslims, who make up more than 90 per cent of the 2 million people in Yala, Pattani, Narathiwat and Songkhla complain of being treated as second-class citizens in mainly Buddhist Thailand
Area was semi-autonomous Islamic Malay sultanate until annexed by Thailand in 1902
Malay-Muslims complain assimilation policies have restricted their customs
Several violent uprisings have been put down by army over the century
Latest uprising flared in 2004 after three years of tough policies on the south by Thaksin Shinawatra, the then premier
Despite martial law imposed in 2004, near daily attacks blamed on Muslim fighters have left about 2,600 people dead and many injured, including Muslims
One of deadliest incidents happened at Krue Se mosque in Pattani in April 2004, where an army raid left 32 men dead.
The soldiers were returning from a two-month break to resume duty in southern Thailand when the explosion destroyed the front end of their bus wounding the soldiers, four seriously.
Police said attackers had hidden explosives in fire extinguishers and buried them in the road, setting them off by remote control.
Security forces raided nearby villages to search for the attackers, the Bangkok Post reported.
In other attacks in the region, a Muslim religious teacher was shot dead in his own house in Yala on Tuesday night.
In Narathiwat, a Muslim man who volunteered with the local defence force was killed in a drive-by shooting late on Wednesday, police said.
Also in Narathiwat, security forces seized guns and bomb-making material and arrested eight suspected fighters in a raid on Tuesday night.
More than 2,600 people have died since a separatist struggle in the Thailand's south flared up in 2004.