Bangkok, Dec 14 (IANS): Three people were killed and 39 others were injured in a bomb explosion at an annual festival in Thailand's Tak province, authorities said on Saturday.

In a social media post, Prime Minister Paetongtarn Shinawatra expressed her deepest condolences to the families of the deceased and the injured in the explosion that occurred shortly before midnight on Friday.

Paetongtarn ordered the police and security agencies to swiftly investigate the incident, apprehend the perpetrators and bring them to justice, as well as instruct authorities to enhance security measures at all upcoming festivals to ensure public safety, Xinhua news agency reported.

Earlier this year, two school children were among eight people injured by shrapnel from a roadside bomb near a park in Thailand's restive southern province of Songkhla.

Among the injured were a district assistant chief, a defence volunteer and villagers.

The Thai Army's Region 4 Forward Command said in February that it believed that southern insurgents detonated the device when a converted pickup truck passed the spot. The explosion blew the vehicle off the road.

The explosion came after a clash between the Thai Army and a group of insurgents. Five suspected southern insurgents were shot dead by security forces on a mountain in Narathiwat Province.

After the clash, Col. Pramote Prom-in, spokesman of the army's southern command, warned of possible imminent retaliation from insurgent groups.

The separatist insurgency in Thailand's largely ethnic Malay-Muslim provinces of Yala, Pattani and Narathiwat has killed nearly 7,000 people since 2004.

The Thai government is in negotiation with several insurgent groups for peace talks.