The images were released as Iraqi prime minister Nouri al-Maliki vowed to "liberate every inch" of territory captured by the insurgents.
The pictures on a militant website appear to show masked fighters of the Islamic State of Iraq and the Levant (Isis) loading the captives onto flatbed trucks before forcing them to lie face-down in a shallow ditch with their arms tied behind their backs.
The final images show the bodies of captives soaked in blood after being shot.
Chief 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 Isis.
He told reporters an examination of the images by military experts showed that about 170 soldiers were shot to death by the militants after their capture.
Captions on the photos showing the soldiers after they were shot say "hundreds have been liquidated," but the total could not immediately be verified.
US State Department spokeswoman Jen Psaki said the Isis militants' claim of killing the Iraqi troops "is horrifying and a true depiction of the bloodlust that those terrorists represent".
She added that a claim that 1,700 were killed could not be confirmed by the US.
UN human rights chief Navi Pillay warned against "murder of all kinds" and other war crimes in Iraq, saying the number killed in recent days may run into the hundreds.
She said in a statement that her office had 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 Isis militants overran Iraqi cities and towns, she said.
The grisly images could sap the morale of Iraq's security forces, but they could also heighten sectarian tensions.
Thousands of Shias are already heeding a call from their most revered spiritual leader to take up arms against the Sunni militants who have swept across the north in the worst instability in Iraq since the US withdrawal in 2011.
Isis has vowed to take the battle to Baghdad and cities farther south housing revered Shia shrines.
Although the government bolstered defences around Baghdad, a series of explosions inside the capital killed at least 19 people and wounded more than 40, police and hospital officials said.
Security at the US Embassy was strengthened and some staff members sent elsewhere in Iraq and to neighbouring Jordan, the State Department said. A military official said about 150 Marines have been sent to Baghdad to help with embassy security.
While the city of seven million is not in any immediate danger of falling to the militants, food prices have risen - twofold in some cases - because of transportation disruptions on the main road heading north from the capital. The city is under a night-time curfew that begins at 10pm.
In a fiery speech to volunteers south of Baghdad, prime minister Nouri al-Maliki vowed to regain territory captured last week by Isis.
"We will march and liberate every inch they defaced, from the country's northernmost point to the southernmost point," he said.
On Saturday, hundreds of armed Shia men paraded through the streets of Baghdad in response to a call by Grand Ayatollah Ali al-Sistani for Iraqis to defend their country. Isis has vowed to attack Baghdad but its advance to the south seems to have stalled in recent days. Thousands of Shias have also volunteered to join the fight against the Isis, also in response to al-Sistani's call.
Armed police, including SWAT teams, were seen at checkpoints in Baghdad, searching vehicles and checking drivers' documents. Security was particularly tightened on the northern and western approaches, the likely targets of Isis fighters on the capital.
According to police and hospital officials, a car bomb in the city centre killed 10 and wounded 21. After nightfall, another explosion hit the area, killing two and wounding five. A third went off near a falafel shop in the sprawling Sadr City district, killing three and wounding seven. And late on Sunday, a fourth blast in the northern Sulaikh district killed four and wounded 12.
Suicide and car bombings in recent months have mostly targeting Shia neighbourhoods or security forces.
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