Most of the soldiers are in civilian clothes. Some have military uniforms underneath, indicating they may have hastily tried to disguise themselves as civilians in a failed bid to escape the al-Qaeda-linked militants.

The photos - which are undated - then show the prisoners being loaded into the back of crowded flatbed trucks under the gaze of armed militants.

They are then taken to a field where they are forced to lie face down in a shallow ditch, their arms tied behind their backs.

In the next photo, a line of militiamen armed with automatic weapons takes aim at the prisoners, as one man appears to beg for his life.

Another photo shows a militiaman executing the prisoners in a hail of gunfire from his Kalashnikov that kicks up the dirt around their bodies. The final images show the prisoners' blood-soaked bodies.

The photos appear to show at least three separate places in which men were executed and possibly a fourth.

"This is the fate of the Shiites which Nouri [Prime Minister Nouri al-Maliki] brought to fight the Sunnis," a caption to one of the photos reads.

The captions said the killings were to avenge the death of an ISIL commander, Abdul-Rahman al-Beilawy, who was killed shortly before the group captured Iraq's second largest city Mosul and Saddam Hussein's hometown of Tikrit in a lightning offensive.

US-trained Iraqi forces folded immediately as ISIL extremists captured key towns in swift succession last week, abandoning vehicles and positions and discarding their uniforms.

Iraq's top military spokesman, Lieutenant General Qassim al-Moussawi, yesterday confirmed the authenticity of the photographs and said he was aware of cases of mass murder of captured Iraqi soldiers in areas held by ISIL.

However, the claims could not be independently verified.

In one caption, the militants themselves said that they had executed hundreds of captured soldiers. The photos that have been posted online showed dozens of bodies, while tweets attributed to ISIL claimed they had killed 1,700 Shia soldiers.

Iraqi officers said their forces were now starting to repel the militants, and that soldiers had recaptured two towns north of Baghdad.

As troops began to drive back the militants, they found the burned bodies of 12 policemen after retaking the town of Ishaqi north of Baghdad, pointing to another atrocity carried out by ISIL militants.

The atrocities 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.

The United States condemned the “horrifying” massacre by Islamic militants.

“The claim by the Islamic State of Iraq and the Levant that it has massacred 1,700 Iraqi Shia air force recruits in Tikrit is horrifying and a true depiction of the bloodlust that these terrorists represent,” US State Department spokeswoman Jen Psaki said in a statement.

“While we cannot confirm these reports, one of the primary goals of ISIL is to set fear into the hearts of all Iraqis and drive sectarian division among its people.”

Psaki said the US would evacuate some of its staff and boost security at its embassy - America’s largest worldwide, and located in Baghdad’s fortified Green Zone - due to “ongoing instability.”

Australia followed suit, announcing it was withdrawing a number of officials from Baghdad, with only an “essential core” of embassy staff to remain.

Washington has also deployed an aircraft carrier group to the Gulf as US President Barack Obama said he was weighing “all options” on how to support the Iraqi government.

But he has ruled out a return to Iraq for US soldiers, which left the country nearly three years ago after a bloody and costly occupation launched in 2003.

Iran, which supports Iraq’s Shia-led government, has warned against foreign military intervention in the country, voicing confidence that Baghdad can repel the onslaught.

But reports suggest it already has a small number of its Revolutionary Guards in Iraq as military advisers.

Iranian President Hassan Rouhani said on Saturday that Iraq had not asked for his country’s help. But in surprise comments, he added that Iran may “think about” cooperating with archfoe America to fight the militants in Iraq.

US officials told The Wall Street Journal that the Obama administration may use nuclear talks starting in Vienna on Monday to discuss the Iraq crisis with Iran.

Iraqi Prime Minister Nuri al-Maliki’s security spokesman, Lieutenant General Qassem Atta, said on Sunday that Baghdad’s forces have “regained the initiative” and killed 279 “terrorists” in the past 24 hours.

There was no way of independently verifying those assertions, however. Iraqi officials often announce large militant death tolls and downplay their own casualties.

Officials added that security forces and tribal fighters repelled a militant assault in the strategic town of Tal Afar near the Syrian border. It provides a critical corridor for militants to access conflict-hit Syria.

Ten people were killed in militant shelling of the town, and 18 anti-government fighters also died in ensuing clashes.

And militants took control of the Al-Adhim area in Diyala province, north of Baghdad, officers said.

Although violence has eased in Baghdad, apparently as militants concentrate their efforts elsewhere, the capital has not been spared, with a Sunday afternoon bombing killing nine people.

Baghdad’s embattled forces will be joined by a flood of volunteers after a call to arms from top Shiite cleric Grand Ayatollah Ali al-Sistani, but a recruitment centre for volunteers came under attack on Sunday, killing six people.

Lakhdar Brahimi, the former UN and Arab League envoy to Syria, said the international community’s neglect of the conflict in Syria had precipitated the Iraq crisis.

“It is a well-known rule: a conflict of this kind (in Syria) cannot stay confined within the borders of one country,” said Brahimi, who resigned as UN-Arab League representative to Syria last month.

UN human rights chief Navi Pillay had earlier warned of "murder of all kinds" and other war crimes as the jihadists advanced across the north of Iraq. She said the situation in Iraq was deteriorating rapidly amid reports of "summary executions and extrajudicial killings and the massive displacement of an additional half a million people".

"Murder of all kinds, mutilation, cruel treatment and torture constitute war crimes," Pillay said in a statement.

The UN has received reports of the suicide in Mosul of four women who had reportedly either been raped or forced to marry ISIL soldiers.

More lies about Iraq, says Manning

The detained US soldier convicted of leaking a trove of secret documents to WikiLeaks has made a rare foray into public life to warn Americans they were being lied to about Iraq once more.

Chelsea Manning is serving 35 years on espionage charges.

"I understand my actions violated the law. However, the concerns that motivated me have not been resolved," the soldier formerly known as Bradley Manning wrote in a New York Times editorial.

"As Iraq erupts in civil war and America again contemplates intervention, that unfinished business should give new urgency to the question of how the United States military controlled the media coverage of its long involvement there and in Afghanistan."

While the US military was upbeat in its public outlook on the 2010 Iraqi elections, suggesting it had helped bring stability and democracy to the country, "those of us stationed there were acutely aware of a more complicated reality", Manning wrote. He was "shocked by our military's complicity in the corruption of that election. Yet these deeply troubling details flew under the American media's radar."