The Islamic militants who overran cities and towns in Iraq last week posted graphic photos that appeared to show their gunmen massacring scores of captured Iraqi soldiers, while the prime minister vowed Sunday to "liberate every inch" of captured territory.

The pictures on a militant website appear to show masked fighters of the Islamic State of Iraq and Syria, 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. The final images show the bodies of the captives soaked in blood after being shot at several locations.

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.

Captions on the photos showing the soldiers after they were shot say "hundreds have been liquidated," but the total could not immediately be verified.

U.S. 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 U.S.

This image posted on a militant website appears to show militants from the al-Qaeda-inspired Islamic State of Iraq and Syria (ISIS) leading away captured Iraqi soldiers dressed in Tikrit, Iraq. (Associated Press) On Friday, 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 U.S. withdrawal in 2011.

ISIS has vowed to take the battle to Baghdad and cities farther south housing revered Shia shrines.

Defences bolstered around Baghdad

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 U.S. 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 7 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 nighttime curfew that begins at 10 p.m.

In a fiery speech to volunteers south of Baghdad, Prime Minister Nouri al-Maliki vowed to regain territory captured last week by the ISIS.

"We will march and liberate every inch they defaced, from the country's northernmost point to the southernmost point," he said. The volunteers responded with Shia chants.

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.

The city looked gloomy Sunday, with thin traffic and few shoppers in commercial areas. At a popular park along the Tigris River, only a fraction of the thousands who usually head there were present in the 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.

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

The officials spoke on condition of anonymity because they were not authorized to speak to the media.

Suicide and car bombings in recent months have mostly targeting Shia neighborhoods or security forces.

Psaki said in a statement that much of the embassy staff will remain even as parts of Iraq experience instability and violence.

"Overall, a substantial majority of the U.S. Embassy presence in Iraq will remain in place and the embassy will be fully equipped to carry out its national security mission," she said.

Some staff was temporarily moved elsewhere in Iraq and to Jordan, she said.

Secretary of State John Kerry called foreign ministers in Jordan, the United Arab Emirates, Saudi Arabia and Qatar to discuss the need for Iraqi leaders to work together.

U.S. ships arrive in Persian Gulf

The USS George H.W. Bush arrived in the Persian Gulf as President Barack Obama considers possible military options, although he has ruled out putting American troops on the ground in Iraq.

The U.S. ordered the aircraft carrier USS George H.W. Bush, pictured in April, to the Persian Gulf. (Specialist 3rd Class Abe McNatt/U.S. Navy/The Associated Press) Pentagon spokesman Rear Adm. John Kirby has said the move will give Obama additional flexibility if military action were required to protect American citizens and interests in Iraq.

In neighbouring Iran, the acting commander of the Islamic Republic's army ground forces, Gen. Kiomars Heidari, said Iran has increased its defences along its western border with Iraq, though there was no immediate threat to the frontier.

Iraqi government officials said ISIS fighters were trying to capture the city of Tal Afar in the north and firing rockets seized from military arms depots. The officials said the local garrison suffered heavy casualties and the main hospital was unable to cope with the 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, the military spokesman, confirmed fighting was raging at Tal Afar, but indicated that the militants were suffering heavy casualties. On all fronts north of Baghdad, 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 large part of northern Iraq last week, including the second-largest city of 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 extremist group, which also fights in Syria.

The photos of the Iraqi soldiers purported to have been killed did not provide a date or location, but al-Moussawi said the killings took place in Salahuddin province. Its capital is Tikrit.

Executions avenged ISIS commander death

The photo captions said their deaths were to avenge the killing of an ISIS commander, Abdul-Rahman al-Beilawy. His death was reported by both the government and ISIL shortly before the al-Qaida splinter group's lightning offensive.

"This is the fate that awaits the Shias sent by Nouri to fight the Sunnis," one caption read, apparently referring to al-Maliki.

Most of the soldiers in the photos were in civilian clothes. Some were shown wearing military uniforms underneath, indicating they may have hastily disguised themselves as civilians to try to escape.

Some of the soldiers appeared to be pleading for their lives; others seemed terrified.

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

Different translations for ISIS: The name in the original is the Islamic State of Iraq and "al-Sham" — an Arabic word that refers to Greater Syria or the Levant. So it is alternately referred to as the Islamic State of Iraq and the Levant, or ISIL, or the Islamic State of Iraq and Syria, or ISIS; or other versions of "al-Sham." — The Associated Press

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 AP 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 cut off altogether, he said. No timeframe was given for the shutdowns.