Footage has emerged which appears to show an Islamic State in Iraq and Syria (ISIS) fighter questioning and killing Iraqi soldiers.
It comes after the insurgents, who have seized large swathes of territory in the past week, released graphic pictures purporting to show some of the 1,700 Shia soldiers they claim have shot dead near Tikrit.
According to a translation of the first clip, militants order their prisoners to chant the ISIS slogan "Baqiya", which is thought to mean "(ISIS) will remain in existence" or "Islamic state will stay".
When asked where the government forces are, a soldier replies they have left.
Two of the soldiers ISIS apparently captured
The fighters then threaten to chop off the head of one of the soldiers.
The earlier images posted online showed the Sunni insurgents loading captives on to flatbed trucks, forcing them to lie face-down in a ditch with their arms tied behind their backs before they were shot dead.
The militants claimed to have captured 2,500 soldiers, although that number has not been verified.
Chief Iraqi military spokesman, Lieutenant General Qassim al Moussawi, confirmed the authenticity of the earlier pictures and said he was aware of cases of mass murder of captured soldiers in areas held by ISIS.
Militants have posted images purportedly showing the massacre of soldiers
But he added an examination of the images by experts showed that approximately 170 soldiers were shot dead, not the 1,700 ISIS had claimed.
Jen Psaki, spokeswoman for the US State Department, condemned the "bloodlust" of ISIS after the pictures emerged.
Sky's Diplomatic Editor Tim Marshall said the release of the images means the militants have now "got the attention of the world".
Meanwhile, Prime Minister Nouri al Maliki has vowed to recapture the territory seized by the insurgents last week.
Fighters have swept through towns and cities along the Tigris, but have since halted around an hour's drive north of the capital, Baghdad.
In the latest fighting, residents of the northern town of Tal Afar said ISIS had taken control after a dawn raid.
Militants have also seized the Al-Adhim area in Diyala province, north of Baghdad.
Iraq's army is holding out in Samarra, which has been the subject of numerous attacks by militants.
A convoy travelling there to reinforce troops was ambushed by Sunni fighters on Sunday near the town of Ishaqi.
Peshmerga soldiers on patrol in northern Iraq
Security in Baghdad has been tightened, but despite this three explosions left at least 15 people dead and dozens injured.
As the conflict continues, US Secretary of State John Kerry revealed the US is "open to discussions" with Iran over the crisis.
He added drone strikes are an option in combating ISIS's offensive.
Foreign Secretary William Hague has spoken with his Iranian counterpart about the conflict. But, the Foreign Office declined to disclose the call's contents.
The Government has banned ISIS under UK terrorism laws, making membership and support of the extremist group a criminal offence.