YALA - Gunmen in insurgency-wracked southern Thailand killed 15 village defense volunteers and wounded five security personnel in what is believed to be the deadliest attack on government forces since the separatist rebellion began 15 years ago. Blood-stained clothing found at the scene suggested that some of the attackers on two checkpoints in Yala province late Tuesday night also may have been wounded in exchanges of gunfire, said army spokesman Col. Kiattisak Neewong. He said four of the slain officers were women and one was a doctor. In what appeared to be coordinated actions, nails were scattered on a highway to disable vehicles entering Yala, a small explosive device was found placed near an electrical pole to knock out power, and several burning tires were left at a school, said Col. Thaweesak Thongsongsi, a Yala police superintendent. Thai Prime Minister Prayuth Chan-ocha said Wednesday the attack may show the insurgents are switching their focus from attacking soldiers and police to the volunteers, who are lightly armed soft targets. He said plans would have to be drawn up to better protect them. Thailand’s volunteer forces in the south are raised from villages and receive weapons training from the army but no salary. They are usually issued shotguns but often carry personal handguns, and only guard their own villages rather than seek to confront the insurgents.