Iraq's security forces and allied Shiite militias executed at least 255 Sunni prisoners as they fled a lightning jihadist-led advance last month, Human Rights Watch said on Friday.

"Iraqi security forces and militias affiliated with the government appear to have unlawfully executed at least 255 prisoners... since June 9," the watchdog said in a statement.

"The mass extrajudicial killings may be evidence of war crimes or crimes against humanity," the New York-based HRW said.

It said the killings appeared to have been carried out in revenge for the onslaught led by what was still known last month as the Islamic State in Iraq and the Levant (ISIL).

The group, which has since rebranded itself as the Islamic State (IS), is a Sunni extremist organisation which last month overran large swathes of Iraq, including second city Mosul, and has since declared a "caliphate" straddling the border with Syria.

"Gunning down prisoners is an outrageous violation of international law," said HRW's deputy Middle East director, Joe Stork.

"While the world rightly denounces the atrocious acts of (ISIL), it should not turn a blind eye to sectarian killing sprees by government and pro-government forces."

The rights group said it had documented massacres of prisoners last month in Mosul, as well as in the towns and villages of Tal Afar, Baquba, Jumarkhe and Rawa.

"In one case the killers also set dozens of prisoners on fire, and in two cases they threw grenades into cells," HRW said.

It demanded an international investigation into the killings.

Saddam deputy praises jihadist 'heroes' in unverified message
Baghdad (AFP) July 12, 2014 - The wanted deputy of executed dictator Saddam Hussein praised the Islamic State jihadists who took over large swathes of the country last month as heroes in an unauthenticated audio message released on Saturday.

The recording features a 15-minute speech in a raspy, quavering voice purported to be that of 72-year-old Izzat Ibrahim al-Duri, who was Iraq's vice-president when US-led coalition forces invaded in 2003.

The voice in the recording, which AFP could not immediately confirm to be that of Duri, praised "some groups of (insurgents) Ansar al-Sunna and, in addition to these, the heroes and knights of Al-Qaeda and the Islamic State."

The Islamic State (IS) has been fighting in Syria and Iraq and on June 29 proclaimed a "caliphate" straddling both countries and headed by Abu Bakr al-Baghdadi, who now calls himself Caliph Ibrahim.

Its fighters spearheaded a devastating military offensive by a coalition of Sunni militant groups that swept through large swathes of northern and western Iraq.

The onslaught was contained barely 50 miles from the capital Baghdad, exacerbating sectarian tensions nationwide and pushing Iraq to the brink of disintegration.

"We give them a special salute with pride, appreciation and love," said the man in the recording, introduced by another voice as the great commander of the Baath party.

"A dear salute to their leaders, which issued a general amnesty on every one who betrayed himself, betrayed God, betrayed his country but then atoned."

He then went on to list several, sometimes obscure, Sunni militant groups believed to have rallied behind the Islamic State for last month's offensive.

Saddam's regime was secular and Izzat al-Duri is believed to be the leader of Jaysh Rijal al-Tariq al-Naqshbandi (JRTN), or Naqshbandiya order, a group of Sufi inspiration long seen as a rival to jihadist groups such as IS.

The latest such message attributed to Saddam's red-haired right-hand man, one of the former regime's most recognisable figures, was released in January 2013.

After the December 2003 capture of Saddam Hussein, the wiry general nicknamed "Red Moustache" by some became the most senior figure, the King of Clubs, in the US army's infamous deck of cards of wanted Iraqis.

He was best known to Iraqis as "The Iceman" for his humble origins selling blocks of ice on the streets of Mosul, Iraq's second city and now a key jihadist hub.

Duri rose to become the number two official in the Revolution Command Council in Saddam's regime and was seen as one of the few men in the country who could stand up to the former dictator.

He was suffering from leukemia at the time of the invasion and thought by many to have died.

But his name resurfaced in unverified recordings linked to the JRTN, which appears to have evolved from a network of influential Sunnis akin to freemasonry into a fully-fledged armed rebel group bent on undermining the Shiite majority's stranglehold on power.