Two mayors and a vice-mayor were gunned down by unidentified assailants in the Philippines last week.

One of the mayors had been on President Rodrigo Duterte's watchlist of local officials suspected of having links to the narcotics trade, but the other two victims had no known drug ties. One was even an ally of Mr Duterte.

Government critics suggested that the killings could have been orchestrated to create a "climate of fear" that will give Mr Duterte a reason to place the country under martial rule.

Watchdogs are concerned that the spate of attacks on local officials could signal the start of a deadly election season, stoked by Mr Duterte's support for vigilante justice. Mid-term elections are slated for May next year. Police said the vice-mayor was killed a day after he revealed plans to run for mayor.

The Philippines is one of the world's most dangerous places to be an elected official or a political candidate. In 2016, 30 people were killed in the run-up to the general election. On election day itself, 10 more died. This year, police tallied 33 deaths in a two-month campaign season to elect officials of 42,000 villages - 22 were candidates.

Human rights groups are worried that as the nation is being primed for the May elections, the death toll could be far higher this time. "The difference here is the way Duterte has threatened officials with death, and then they are being killed one after the other," said Human Rights Watch researcher Carlos Conde.

The latest attacks brought to 15 the number of mayors and vice-mayors killed since Mr Duterte unleashed his anti-narcotics campaign in 2016.

Mr Wilnor Papa, Philippine spokesman for Amnesty International, said that while political violence is not new, "Duterte has aggravated it through his pronouncements".

Vice-President Leni Robredo said yesterday: "It has become ordinary now to see someone killed every day. It speaks of a culture of violence."