The blast occurred a day after an army patrol in neighbouring Narathiwat province was ambushed, leaving eight soldiers dead, one of them beheaded, in the worst attack on the Thai military in the south since June last year.
Muslims, who make up more than 90 per cent of the population in Yala, Pattani, Narathiwat and Songkhla provinces 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 PM
Despite martial law imposed in 2004, near daily attacks blamed on Muslim fighters have left about 2,800 people dead and many injured, including Muslims
An army spokesman called the ambush a reaction to what he claimed was the military's progress in tracking down leaders of an uprising in the mostly-Muslim south.
Surayud Chulanont, the Thai prime minister, tried to downplay any apparent upsurge in violence.
"This kind of clash can happen any time. It is not a serious escalation," he said.
Attacks in the south have declined since August, down by almost half to about 20 recorded incidents in the September to November period, according to the Deep South Watch centre at Pattani's Prince of Songkhla University.
But Monday's ambush came just an hour after a roadside bomb targeted another convoy protecting teachers in Yala province, injuring two of the soldiers, local police said.
And attacks in the region have grown increasingly brutal in recent months, with victims beheaded, mutilated and even crucified.
The military and police have also come under a cloud of suspicion, with the army saying it might have spies supplying fighters with information and seven police officers under investigation, the Bangkok Post reported.
On Sunday, six suspected fighters escaped from Tanyong police station in Narathiwat. The fugitives had been in custody for several days awaiting trial for more than 10 cases of violence.