A powerful bomb killed six people and injured twenty Tuesday in one of the deadliest attacks in recent months in Thailand's insurgency-plagued south, an army spokesman said. An explosive, planted in a motorcycle gas tank, was detonated as two suspected Muslim insurgents rode the motorcycle past a roadside restaurant, Army spokesman Col. Akara Thiprote said in a telephone interview. "We suspect that the bomb went off in front of the restaurant before they intended it to," Akara said, adding that the two were killed instantly. Two victims were restaurant workers and the other two were people standing outside the open-air restaurant. The attacks came after a lull in years of violence in Thailand's southernmost provinces. More than 2,600 people have been killed in the Muslim-majority provinces of Yala, Pattani and Narathiwat, and the southern part of nearby Songkhla, since a long-simmering Islamic separatist insurgency flared up in January 2004. A recent lull in the large-scale violence is partly due to a targeted effort to round up major suspects, Akara said. Despite government efforts to suppress the rebels, they carry out drive-by killings and small-scale bombings almost daily, apparently in an effort to terrorize Buddhist residents into leaving the area. The attack came on the eve of the 80th birthday of Thailand's beloved monarch but there was no immediate link between the two events. Thailand's aging King Bhumibol Adulyadej pleaded for his people to be united to safeguard the nation in his annual birthday speech. He did not talk about the violent Islamic insurgency in the three southernmost provinces.