SULTAN KUDARAT- Two journalists have been shot dead in two days in separate incidents in the Philippines, reinforcing the country's image as one of the world's most dangerous for media workers, officials said Monday.

Broadcaster Rudy Alicaway and columnist Leo Diaz were the third and fourth journalists to be killed since President Rodrigo Duterte, a vocal critic of the press, took office last year, said Dabet Panelo, secretary-general of the National Union of Journalists of the Philippines (NUJP).

In both cases the victims were riding motorcycles when gunmen on another motorcycle came up behind and shot them dead.

Diaz, 60, a correspondent of Manila tabloid Balita (News) and a columnist for a community newspaper, was killed in the southern town of President Quirino in Sultan Kudarat on Monday, said Fort Yerro, a Balita consultant.

Diaz, a former policeman, had covered stories on political corruption, illegal gambling and drugs, said Yerro, but added that he did not know of any threats against him.

Alicaway, 47, who hosted a weekly community affairs show on DXPB radio station, was killed in the town of Molave, Zamboanga del Sur on Sunday, a police report said.

Rocel Navarro, manager of the government-run station where the victim's show aired, said he had not covered controversial issues.

"We are asking ourselves why this happened to him. He had no enemies as a broadcaster," Navarro told AFP.

Both killings took place on the southern island of Mindanao, where Islamic militants, communist guerrillas and political warlords are active.

"This again highlights the culture of impunity, in the attacks against and killings of Filipino journalists that have remained unabated despite an international outcry," the NUJP said.

Last week Michael Marasigan, a respected former newspaper editor, was shot dead in a Manila suburb.

A study by the International Federation of Journalists said last year that 146 journalists were killed between 1990 and 2015, making the Philippines the second most dangerous country for the media after Iraq.