SUVA, Fiji (AFP) - Amnesty International on Thursday (Oct 22) called for a major investigation into claims of torture in Fiji, after four security personnel were accused of sexually assaulting a prisoner in the South Pacific nation.

Prosecutors on Wednesday said four men - three police and one soldier - had been charged over an alleged 2012 attack on an escaped prisoner that was filmed and posted online, prompting outrage.

The rights watchdog said the attack "clearly amounted to torture" and called for an independent investigation into allegations of abuses by Fiji's security forces.

"This video reveals the shocking level of brutality that the police force and members of the military are capable of," Amnesty's Pacific campaign director Josef Benedict said in a statement.

Fiji was effectively under martial law from 2006, when the military staged a coup, to September last year, when long-awaited elections were finally held.

Mr Benedict said Amnesty had multiple reports of security forces using torture and ill-treatment, which needed to be investigated as thoroughly as the video case.

"It's been a long wait for justice for this torture victim, but many others are still waiting for their complaints to be investigated," he said.

"Justice must be done in all these cases to assure people in Fiji that ill-treatment and torture is no longer tolerated."

The four men charged appeared briefly in Nasinu Magistrates Court, where they were remanded in custody to reappear on Oct 28. Police are searching for a fifth suspect, a soldier.