Human Rights Watch has confirmed a report about the killings of scores of police and soldiers by Sunni militants in Iraq in the days after it captured the northern city of Mosul on June 10.
The execution-style killings were widely reported after graphic photos emerged on the internet showing dozens of men wearing civilian clothes lined up and bent over as militants pointed rifles at them from behind. A final set of photos showed bodies.
Providing new details, Human Rights Watch said fighters from the Islamic State of Iraq and the Levant (Isis) killed 160 to 190 men in two locations in Saddam Hussein's home town of Tikrit between June 11 and 14.
"The number of victims may well be much higher, but the difficulty of locating bodies and accessing the area has prevented a full investigation," the group said.
Human Rights Watch said it used satellite imagery from last year and publicly available photos taken earlier to pinpoint the site of the killings in a field next to the Tigris River and near one of Saddam's former palaces.
It said satellite imagery of the site from June 16 did not reveal bodies but showed indications of earth movement consistent with the two shallow trenches visible in the photos.
Peter Bouckaert, emergencies director at Human Rights Watch, said: "The photos and satellite images from Tikrit provide strong evidence of a horrible war crime that needs further investigation."
Chief Iraqi military spokesman Lieutenant General Qassim al-Moussawi confirmed the photos' authenticity on June 15 after they first surfaced, and said at the time that an examination of the images by military experts showed that about 170 soldiers were shot dead after their capture.
Captions on the photos showing the soldiers after they were shot say "hundreds have been liquidated", but the total could not be verified.
The massacre appeared to be aimed at instilling fear in Iraq's demoralised armed forces as well as the country's Shiite majority, whom Isis views as apostates.
Meanwhile, Iraq's top Shiite cleric has stepped up the pressure on deeply divided political blocs, calling on them to agree on the next prime minister before the newly elected parliament convenes next week to pave the way for an inclusive government.
The appeal by the Iranian-born grand ayatollah Ali al-Sistani comes as current leader Nouri al-Maliki is fighting to keep his job, with his former Shiite allies and even key patron Iran exploring alternatives in the face of Iraq's worst crisis since US troops withdrew at the end of 2011.
Mr al-Maliki, who has governed the country since 2006, needs support from other parties after his State of Law bloc won the most seats in the elections but failed to gain the majority needed to govern alone.
The process of forming a government in Iraq is always protracted but it has been further complicated by the lightning advance by the al Qaida breakaway group Isis in northern and western Iraq.
Isis's stunning gains were made possible as Iraqi security forces melted away in large part due to fear of the insurgents' brutality.
On the military front, a senior Iraqi army official said four helicopters carrying Iraqi commandos landed at a football pitch inside a university campus in militant-held Tikrit last night and clashed with Isis fighters for several hours.
One of the helicopters developed mechanical problems after takeoff from Tikrit, 80 miles north of Baghdad, but landed safely in the provincial military headquarters. The official had no word on casualties and declined to specify the mission's objectives.
The official also said 200 troops had arrived at a key refinery north of Baghdad under attack by militants for more than a week. The reinforcing troops join a 100-strong contingent that has been defending the Beiji refinery, Iraq's largest and the source of about a quarter of the country's oil product needs, including fuel for power stations.
State-run television aired footage today purporting to show troops disembarking from helicopters at Beiji, with some carrying boxes of supplies. Dense black smoke was rising from what appeared to be a large fuel tank.
The news from Tikrit and Beiji suggested that Iraq's military was stepping up efforts to regain its footing against the insurgents, who appear to be trying to carve out a self-styled Islamic state astride the Iraqi-Syrian border.
Mr al-Maliki today warned army commanders that militants were likely to try to undermine security in the Iraqi capital ahead of Tuesday's parliamentary session. "Baghdad must be secured and not subjected to any instability at this time," he said in televised comments.
He sounded upbeat on the military situation, saying the armed forces had regained the initiative and were on the offensive. He also vowed to severely punish army commanders whom he said were taking bribes from soldiers to stay home.
The US and other world powers have pressed Mr al-Maliki, in office since 2006, to reach out to the country's Sunni and Kurdish minorities and have called for a more inclusive government that can address long-standing grievances.
But he has instead widely been accused of monopolising power and alienating Sunnis, and his failure to promote national reconciliation has been blamed for fuelling the Sunni anger.
Mr al-Sistani also called on Iraq's politicians to agree on the next parliament speaker and president by the time the new legislature meets on Tuesday, a cleric who represents him told worshippers in a sermon in the holy city of Karbala.