A former town mayor in the Philippines and his brother have been killed in separate shootouts with the police, becoming the latest victims of President Rodrigo Duterte's so-called war on drugs.

Talib Abo, ex-mayor of Parang town in Maguindanao province, was shot in his home by officers on Friday during an anti-drug operation in the southern Cotabato City. His brother Bobby was also killed in a similar pre-dawn raid.

Police and anti-drug agents were serving search warrants when Abo and his brother allegedly resisted and shot at the authorities.

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"They resisted and shot it out with the law enforcers," Senior Superintendent Rolly Octavio, the city police director, told local media.

Duterte had accused Abo of being a narco-politician as far back as 2006, linking him to the transport of 3.5kgs of crystal methamphetamine, locally called shabu, to Davao City in an ambulance.

Abo had denied the allegations and claimed that political rivals were behind the accusations.

Since taking office in June 2016, the brash-talking Duterte expanded nationwide a deadly campaign against illegal drugs that he had implemented as a long-time mayor of the southern city of Davao.

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According to police, the current campaign has left close to 5,000 suspects dead, including nearly a dozen mayors, in what they claimed were clashes with law enforcement.

But rights groups and opposition politicians say more than 20,000 people have been killed by police since 2016.

The slain suspects have included several incumbent and former local government officials who have been accused of providing protection to illegal drug syndicates in their areas.