PORT-AU-PRINCE, HAITI (AFP) - Fifteen people were shot to death this week in Haiti, including a journalist and an opposition activist, in a terrorist attack, the country's prime minister said on Thursday (July 1).

The victims were slain in the capital Port-au-Prince in violence that authorities initially blamed on revenge attacks after the death of a police officer.

Violence has been sharply on the rise in Haiti this year, with gun fights between rival groups prompting many residents of poor districts of the city to flee their homes.

"We are appalled at the terrorist attack that has been committed by a terrorist group calling themselves Fantom 509," Haitian Prime Minister Claude Joseph said.

The group emerged in March when some police officers and former Haitian National Police members got together to call for the founding of a union - a demand that was subsequently granted.

Hours before the overnight shooting spree, police union spokesman Guerby Geffrard was killed in Port-au-Prince.

Haitian Police Chief Leon Charles initially said the shootings - whose victims included reporter Diego Charles and political activist Antoinette Duclair - were in revenge for Mr Geffrard's killing.

"It was Fantom 509 who did this," Mr Charles said on Thursday.

Addressing the group directly, he said: "Instead of letting justice do its job, you wanted to carry out your own justice - but the police, the country, and the international community will not accept this."

Authorities have yet to offer proof to support the accusations, and Mr Charles' statements sparked criticism from journalists and civil rights organisations who doubt their truth.

Undermined by insecurity and political instability, Haiti is struggling to emerge from a string of seemingly never-ending crises, which of late have resulted in a surge in kidnappings and gang violence.