A bomb went off on a village road in Yala's Muang district Wednesday, injuring two defence volunteers, police said.
Suthep Chanwanno, 30, and Pinyo Khonthon, 38, were wounded by shrapnel when a three-kilogramme explosive device exploded in tambon Yupo.
The pair was among eight defence volunteers on their way to escort teachers to school. A bomb was planted behind a pile of bricks and detonated when the defence volunteers were riding past it on their motorcycles.
Police suspected the blast was the work of militants headed by Roki Dorloh, who are active in Ban Thung Yamu and Ban Yupo.
Yala police chief Pol Maj Gen Peera Boonliang ordered a security boost around the Muang municipality following the explosion.
Forces in six districts of Songkhla also stepped up surveillance amid reports about possible car bombs.
Checkpoint staff were told to keep an eye out for Isuzu pickup trucks and Malaysian-made Proton cars.
The districts under threat of attack were identified as Muang, Chana, Thepha, Na Thawi, Saba Yoi and Hat Yai.
Meanwhile, police Wednesday identified two people suspected of a grenade attack at a base in Wat Lak in Yala on Tuesday.
Security camera footage showed the assailants wearing female Muslim clothing.
They were identified as Sofwan Samah and Usaman Dorkho.
The men, also wanted in connection with other attacks, are active in Yala's Yaha, Raman and Muang districts, police said. The M26 grenade attack left eight soldiers of the Yala Task Force 11 injured.
Pattani governor Pramuk Lamul said authorities would seek warrants for the arrest of six people suspected of a spate of bomb and arson attacks in Muang district over the weekend.
He said all potential suspects would be identified in a week based on evidence collected from the scenes.
One suspect was identified as Masore Dueramah who is believed to have experience in bomb-making.
The governor said residents were told to be on high alert following the weekend violence. Their cooperation would help strengthen security.
The army chief is visiting the deep South to explain changes in the government's security stance.
Army commander Gen Prayuth Chan-ocha and Gen Yutthasak Sasiprapa, the premier's adviser, visited Pattani Wednesday to explain the enforcement of Section 21 of the Internal Security Act (ISA).
The government decided on Monday to replace the state of emergency in the region with the ISA, which is less harsh.
The government hopes that Section 21 of the ISA will make it easier for militants to turn themselves in and to enter the negotiation process.
In Bangkok, Deputy Prime Minister Chalerm Yubamrung denied Wednesday he sought a meeting with Malaysian ex-premier Mahathir Mohamad to ask him to meditate in talks with insurgents.
He said he wanted to meet the Malaysian's ex-premier to discuss developments in the southern border provinces.
He said he would meet his newly appointed advisers on the deep South on March 1 and would present their opinions to the National Security Council if they had merit.
Mr Chalerm has appointed nine members of the Wadah group of politicians in the far South as his advisers amid criticism that some may have links to insurgents.