NARATHIWAT, THAILAND (BNO NEWS) -- A roadside bomb on Friday killed five defense volunteers and severely injured another in Thailand's restive southern province of Narathiwat, local police said.

The explosion occurred when the rangers were heading to investigate a fatal shooting in a rubber plantation in Narathiwat's Ruso district, according to Thai news agency MCOT. A 20-kilogram (44 pound) homemade bomb hidden beneath the road surface was triggered as the rangers' pickup truck passed a village, killing the five defense volunteers immediately and wounding another.

Local police are investigating the attack, according to the news agency.

Prior to the bombing, a 53-year-old man was gunned down in a rubber plantation in the Ruso district. Police believe the incidents were carried out by suspected members of the insurgent group Runda Kumpulan Kecil (RKK) after local authorities ordered security personnel to monitor the movement of armed forces on a hill near the village where the attacks took place.

In January 2004, Muslim militants raided a military arms depot, killing five soldiers and making off with 350 semi-automatic rifles. After the raid, Thailand's army launched a series of attacks on the separatists in the mainly Muslim deep South.

Separatist groups in the southern regions, which are for a majority Muslim, have brought constant and violent uprisings in the region for the past 6 years. Over 4,300 people have died and more than 7,000 others have been injured during this period.