The blast happened about an hour after a bomb blew up on a crowded passenger bus some 300 kilometers (190 miles) to the west, injuring at least 18 people in the town of Polomolok, officials said.
No group claimed responsibility for either attack. Col. Rolando Joselito Bautista, the military commander in Basilan province, where the second blast occurred, said Abu Sayyaf militants were among the initial suspects in that attack because of their history of deadly bombings in the area, where the al-Qaida-linked group first surfaced in the 1990s.
The roadside bomb struck the two-vehicle convoy of Isabela city Vice Mayor Abdulbaki Ajibon, killing her driver and another passenger, and two pedestrians, said Bautista. Ajibon was uninjured, although her SUV was badly damaged.
"The vice mayor was shocked but she's safe. Two of her people died in this attack," Isabela police chief Supt. Albert Larubis told The Associated Press by telephone.
The bomb was packed with shrapnel and shattered the glass of nearby buildings, he said. It occurred in front of the house of Mayor Cherrylyn Santos-Akbar, who was not home, he said. Three years ago, a roadside bomb exploded as Ajibon's convoy was passing in the same area that was hit by Thursday's bombing, although no one was killed or wounded in that past attack, police said.
Ajibon was reportedly aspiring for a higher office in next year's elections and investigators were also trying to determine if the attack had something to do with the often-violent political rivalries in the south.
The earlier bomb on the bus appeared to have been placed in the overhead baggage bin, officials said. The injured were rushed to nearby hospitals in General Santos city, in South Cotabato province. They included a 9-year-old girl with a serious head wound, according to emergency ward nurse Gina Loren.
Past bombings of passenger buses in the south have been blamed on Muslim insurgents and criminal gangs.
The Abu Sayyaf has been designated as a terrorist organization by the United Nations and the Philippines for deadly bombings, extortion, kidnappings for ransom and beheadings.
It's one of at least four Muslim rebel groups outside of a peace deal the government signed with the largest insurgent group, the Moro Islamic Liberation Front, last year in the hope of peacefully settling a decades-old separatist rebellion by minority Muslims in the south of the largely Catholic nation.