MANILA (Reuters) - Gunmen killed a city vice mayor in the Philippines on Saturday, police said, less than a week after two mayors were shot dead in separate incidents, including one President Rodrigo Duterte said may have had links to illegal drugs.

The latest attack brought to 15 the number of mayors and vice mayors killed since Duterte unleashed his deadly anti-narcotics campaign when he came to power in 2016, according to CNN Philippines, citing a tally of its own research team.

Duterte has a watchlist of local officials suspected to have links to illegal drug trade.

The daughter of one of the other victims this week said her father was not involved in drugs and police investigators said they had not received any information linking the other to the trade.

Police have killed more than 4,200 suspected drugs dealers and users since Duterte came to power. Several thousand more have been murdered by unknown gunmen, described by authorities as vigilantes or rival gang members.

Police have denied the allegations by rights groups and critics of the campaign that some of the killings were summary executions, saying they had to use force because the suspects, who were armed, had violently resisted arrest.

Presidential spokesman Harry Roque said the “worrisome incidents of violence involving local politicians” must end and police must “spare no effort in getting to the bottom of this latest crime”.

Alexander Lubigan, vice mayor of Trece Martires City in Cavite, south of Manila, was in a sports utility vehicle when unidentified gunmen aboard another SUV opened fire, police said. His bodyguard was also killed.

The motive and identities of the gunmen in the latest killing are still being investigated, Chief Superintendent Edward Carranza, a police regional director, said.

On Monday, Mayor Antonio Halili of Tanauan City in Batangas, south of Manila, was hit by a bullet in the chest during a flag-raising ceremony.

Halili had gained prominence in 2016 for parading drug suspects in the streets, but he was included in Duterte’s watchlist of local leaders.

Duterte has said the mayor may have had some drug involvement and his campaign to parade suspects was a ploy to convince police he was not engaged in the illegal drug trade. Halili’s daughter denied her father had links to the drugs trade.

On Tuesday, gunmen on a motorcycle killed Mayor Ferdinand Bote of General Tinio town in Nueva Ecija province, but police said he was not linked to the drug trade.