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As Iraq’s government bolstered Baghdad's defenses Sunday, the Islamic militant group that captured two major cities last week posted graphic photos that appeared to show its fighters massacring dozens of captured Iraqi soldiers.

The pictures on a militant website appear to show masked fighters of the Islamic State of Iraq and the Levant, or 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, according to The Associated Press. The final images show the bodies of the captives soaked in blood after being shot.

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 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," State Department Spokeswoman Jen Psaki said Sunday. "While we cannot confirm these reports, one of the primary goals of [ISIS] is to set fear into the hearts of all Iraqis and drive sectarian division among its people. We condemn these tactics in the strongest possible terms and stand in solidarity with the Iraqi people against these horrendous and senseless acts of violence.

Some of the pictures appeared to show some of the soldiers pleading for their lives, others seemed terrified.

All soldiers appeared in their early 20s, with some wearing the jerseys of such European soccer clubs like Manchester United and Barcelona. Some of the militants wore black baggy pants and shirts, many of them had sandals or flip flops.

The captions did not provide a date or location, but Iraq's top military spokesman, Lt. Gen. Qassim al-Moussawi, said the killings took place in Salahuddin province, the capital of which is Tikrit.

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 The Associated Press that an examination of the images by military experts showed that about 170 soldiers were shot to death by the militants after their capture.

But last week, Pentagon spokesman Col. Steve Warren said images released by ISIS that claimed to show captured Blackhawk helicopters and other military equipment were faked and photoshopped.

Iraqi authorities appear to be trying to limit the dissemination of such images and other militant propaganda being shared through social media and to deny the militants their use for operational purposes.

Martin Frank, the CEO of IQ Networks, an Internet service provider in Iraq, told The Associated Press that authorities have ordered multiple social media sites including YouTube, Facebook and Twitter to be blocked. On Sunday, they tightened the restrictions further by telling network operators to halt traffic for virtual private networks, which allow users to bypass Internet filters.

Internet traffic in several areas overrun by militants, including Mosul and Tikrit, was ordered to be cut off altogether, he said. No timeframe was given for the shutdowns.

Meanwhile, Iraq’s government on Sunday bolstered defenses around Baghdad Sunday, 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. ISIS has vowed to attack Baghdad but its advance to the south seems to have stalled in recent days. Thousands of Shiites 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 over the weekend manning checkpoints in Baghdad, searching vehicles and checking drivers' documents. Security was particularly tightened on the northern and western approaches of the city, the likely targets of any advance by ISIS fighters on the capital. The city looked gloomy on Sunday, with thin traffic and few shoppers in commercial areas.

At one popular park along the Tigris river, only a fraction of the thousands who usually head there were present on Sunday evening. In the commercial Karada district in central Baghdad, many of the sidewalk hawkers who sell anything from shoes to toys and clothes were absent.

In Baghdad, Iraqi government officials said ISIS fighters were trying to capture the city of 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.

Al-Moussawi confirmed that fighting was raging at Tal Afar, but indicated that the militants were suffering heavy casualties. On all fronts north of the capital, he said, a total of 297 militants have been killed in the past 24 hours.

There was no way to independently confirm his claims.

ISIS 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 ISIS commander, Abdul-Rahman al-Beilawy, whose death was reported by both the government and ISIS shortly before the Al Qaeda 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.

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 ISIS militants overran Iraqi cities and towns, the statement said.

On Saturday, Iraq's security forces were able to halt the advance of Sunni Islamist militants north of Baghdad.

But a police source in Baghdad confirmed to Fox News that a car bomb in Tayran Square in central Baghdad exploded before 12:00 local time Sunday, killing 10 people and wounding 21 others.

After nightfall, another explosion hit the area, killing two and wounding five. The third went off near a falafel shop in the city's sprawling Sadr City district, killing three and wounding seven.

And late Sunday, a fourth blast in the northern Sulaikh district killed four and wounded 12.

Fighting was also reported outside the town of Tikrit, less than 90 miles north of Baghdad. The Wall Street Journal reported that Iraqi soldiers are using the town of Samarra, about 10 miles south, as a jumping-off point for an assault that would re-take Tikrit, the hometown of former Iraqi dictator Saddam Hussein, from ISIS. The Journal also reported that ISIS fighters were using Saddam's well-reinforced presidential palaces as bases of operations as they fought with security forces in the southern parts of the city.

Elsewhere, The New York Times reported clashes in three cities in Salahuddin Province, just north of the capital, with inconclusive results.

Iraq Prime Minister Nouri al-Maliki's battle to regain control of his own country has been aided by the large numbers of Shiites who have responded to his and al-Sistani 's calls for ordinary citizens to wage war against ISIS in what is quickly becoming a sectarian fight.

"The Iraqi fighter is well known for his courage and valor, he has never been known to be defeated or deserted," Maliki's office said in a statement Saturday.

The Washington Post reported that recruitment centers had been set up in mosques and private homes in Baghdad.

The volunteers were first taken to an assembly center in eastern Baghdad, where they were handed military uniforms, and later went to Taji, home of Iraq's largest military base north of Baghdad, to undergo basic training. State-run television aired footage of the volunteers being drilled, still in their civilian clothes.

Fox News' Justin Fishel and The Associated Press contributed to this report.