The grisly images could further sharpen sectarian tensions as hundreds of Shiites heed a call from their most revered spiritual leader to take up arms against the Sunni militants that have swept across the north. ISIL has vowed to take the battle to Baghdad and cities further south housing revered Shiite shrines.
A car bomb meanwhile exploded in central Baghdad, killing 10 and wounding 21, according to police and hospital officials. Baghdad has seen an escalation in suicide and car bombings in recent months, mostly targeting Shiite neighborhoods or security forces.
Government officials said ISIL fighters were trying to capture Tal Afar in northern Iraq on Sunday and raining down rockets seized last week from military arms depots. The officials said the local garrison suffered heavy casualties and the town's main hospital was unable to cope with the number of wounded, without providing exact numbers.
The officials spoke on condition of anonymity because they were not authorized to speak to reporters. Tal Afar is mainly inhabited by Turkmen, an ethnic minority.
The government meanwhile bolstered its defenses around Baghdad a day after hundreds of Shiite men paraded through the streets with arms in response to a call by Grand Ayatollah Ali al-Sistani for Iraqis to defend their country. ISIL has vowed to attack Baghdad but its advance to the south seems to have stalled in recent days.
ISIL and allied Sunni militants captured a vast swath of northern Iraq last week, including second city Mosul and Saddam Hussein's hometown of Tikrit, as Iraqi troops, many of them armed and trained by the U.S., fled in disarray, surrendering vehicles, weapons and ammunition to the powerful extremist group, which also fights in Syria.
The captions of the photos say the killings were to avenge the killing of an ISIL commander, Abdul-Rahman al-Beilawy, whose death was reported by both the government and ISIL shortly before the al-Qaida splinter group's lightning offensive, which has plunged Iraq into its bloodiest crisis since the withdrawal of U.S. troops in 2011.
"This is the fate that awaits the Shiites sent by Nouri to fight the Sunnis," one caption read, apparently referring to Iraq's Shiite Prime Minister Nouri al-Maliki.
Iraq's top military spokesman, Lt. Gen. Qassim al-Moussawi, confirmed the photos' authenticity and said he was aware of cases of mass murder of captured Iraqi soldiers in areas held by ISIL.
U.N. human rights chief Navi Pillay warned on Friday of "murder of all kinds" and other war crimes in Iraq, and said the number killed in recent days may run into the hundreds, while the wounded could approach 1,000.
Speaking in Geneva, she said her office has received reports that militants rounded up and killed Iraqi soldiers as well as 17 civilians in a single street in Mosul.
Her office also heard of "summary executions and extrajudicial killings" after ISIL militants overran Iraqi cities and towns, the statement said.
Most of the soldiers who appear in the pictures are in civilian clothes. Some are shown wearing military uniforms underneath, indicating they may have hastily disguised themselves as civilians to try to escape.
Many soldiers and policemen left their uniforms and equipment behind as the militants swept into Mosul, Tikrit and surrounding areas.
The captions did not provide a date or location, but al-Moussawi said the killings took place in Salahuddin province, the capital of which is Tikrit.
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