Image caption Images of a grinning Charles Graner abusing Iraqi prisoners caused international outrage

The ringleader of the US military guards who photographed their abuse of suspected Iraqi insurgents at Baghdad's Abu Ghraib prison has been released.

Charles Graner served more than six-and-a-half years of a 10-year sentence, army officials said.

Graner, a former US Army Reserve specialist, was convicted of leading his six-member team in the sexual humiliation of naked prisoners.

Images of the acts emerged in 2004, sparking international outrage.

He said he was breaking down prisoners for interrogation on the orders of military intelligence officers.

His former fiancee and fellow guard Lynndie England served a three-year sentence for her part in the abuse.

US military spokeswoman Rebecca Steed said Graner, who was freed from Fort Leavenworth, Kansas, will be under the supervision of a probation officer until December 2014.

Infamous photographs

Graner and six other members of the Maryland-based 372nd Military Police Company were charged in 2004 with abusing detainees.

Some of them appear in the infamous photographs of prisoners being humiliated and beaten, displaying obvious satisfaction at the acts.

Graner said the actions were part of a plan directed by military intelligence officers to soften up prisoners for interrogation.

He received the longest sentence of those convicted and is the last defendant in the Abu Ghraib case to be released.

Hana Adwar, an Iraqi human rights activist, told the Associated Press news agency that his release would be met with outrage in Iraq.

"He was charged with a crime that shocked the international community," she said.