Three military rangers were killed while a fourth and a local government official were critically wounded in a bomb attack in Thailand's restive south, officials said on Saturday.
Suspected separatist insurgents detonated the bomb packed with around 20 kilos (45 pounds) of explosives in the middle of a road in the Bacho district of troubled Narathiwat province, a military official said.
The four rangers were on patrol with a district chief assistant when the explosion hit their pick-up truck, leaving a huge crater in the road, he said.
Three of the rangers were killed immediately while the fourth and the local official were taken to hospital in a critical condition, he added.
More than 4,000 people have been killed and thousands more wounded since a separatist insurgency erupted in January 2004 in Thailand's southernmost provinces bordering Malaysia.
Tensions have simmered in the mainly Muslim region -- formerly an autonomous Malay Muslim sultanate -- since it was annexed in 1902 by Buddhist-majority Thailand.