The death toll from a powerful bomb that hit a southern Philippine city rose to eight on Tuesday, police said, as officials pointed to political rivalry as a possible motive.

Thirty-six people were caught in Monday's roadside blast, with four killed at the scene and another four dying in local hospitals overnight, city police chief Rolen Balquin said. Earlier, the total number of people caught in the blast had been given as 32.Police investigators are looking at the possibility that the deadly attack was aimed at a convoy carrying a sister of Cotabato city mayor Japal Guiani, the police chief added."Our initial investigation is leading to that conclusion," Balquin told AFP.He said none of the main armed groups in the southern island of Mindanao, a hotbed to decades-old rebellions by minority Muslims, have claimed responsibility for the attack.Police interviewed Cynthia Sayadi, the mayor's sister who is also the city administrator, at her hospital bed where she was treated for minor injuries, said Balquin, who did not disclose what she told blast investigators."Isn't it obvious? Do I have to state the obvious?" Sayadi said in an interview aired by Manila television network ABS-CBN, apparently supporting the police theory.Her brother had won a second three-year term in May, defeating a former Cotabato mayor and senior leader of a Muslim guerrilla group that signed a peace treaty with the national government in 1996.Two of Sayadi's bodyguards were among those killed in the bomb, which was apparently planted in a vehicle parked at the side of the road, Balquin said.However, he said the authorities were not ruling out the possible involvement of Islamic militants or groups opposed to ongoing peace talks between Manila and the Moro Islamic Liberation Front (MILF), Mindanao's main Muslim rebel group.The attack came a month after the United States, Australia and Canada warned their citizens against travelling to Cotabato and two other Mindanao cities.The nature of the threat was not specified but related to "terrorist and insurgent activities", the US embassy had said."We have intensified visibility patrols and are closely monitoring the activities of various threat groups," Balquin said.Cotabato, a Christian-Muslim entrepot of 300,000 people, was tense Tuesday as police and Philippine Marines stepped up patrols and roadside checkpoints, residents told AFP by telephone.Local Philippine Army spokesman Colonel Dickson Hermoso told AFP among those groups being monitored is the Bangsamoro Islamic Freedom Fighters (BIFF), who oppose the MILF peace talks with the government."They are quiet now because we have been able to contain them. But we expect them to be active again in two days' time. They are only capable of hitting soft targets," he added.Three of BIFF's guerrillas were killed last week near Cotabato in clashes after they fired on a petroleum delivery lorry and laid two roadside bombs.The fasting period of Ramadan is officially set to end on Friday for the minority Muslims in the largely Catholic country of 100 million.