Footage of Iraqi soldiers begging for their lives and being forced to swear allegiance to ISIS has emerged, with one being shot and killed after refusing to give his loyalty to the radical jihadists.
According to a Sky News translation of the clip, the insurgents order the prisoners, who have their hands bound, to chant "Baqiya", the group's slogan which means "Islamic state will stay".
One of the men has a rifle put to his throat as he is pressed to the ground by the barrel of the weapon. Another has trickles of blood running down his cheek.
One of the men has a rifle put to his throat but his death is not shown in the clip
Each one in turn is grabbed by the hair and challenged about his beliefs. Filmed on a mobile phone, the soldiers are taunted by the men, with four of them swearing loyalty, but one refuses. A second clip shows his motionless body on the ground, and the fate of the other two men is not known. "I killed a Shia, I killed a Shia," the man is heard shouting.
On Monday, ISIS were hit by mortar shelling as they seize the city of Tal Afar, which lies between Mosul and the Syrian border.
"Residents are gripped by fear and most of them have already left the town to areas held by Kurdish security forces," local resident Hadeer al-Abadi told Associated Press.
World leaders have condemned "horrifying" photos that show Shia soldiers with their heads down and hand bound being led away in packs, then lying in trenches and being shot in the head.
Iran has said it is willing to co-operate with the United States to help Iraq in its fight against the radical Sunni insurgents, with Washington hinting that it too may consider a temporary deal.
President Hassan Rouhani told state television on Saturday that although Iraq had not asked its Western neighbour for help, it stood ready to provide support "within international law". That aid is most likely to include logistics support and weapons, rather than the ground troops which is the one thing that Iraq sorely needs.
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Iran has sent Major General Qassem Suleimani, a veteran of the feared Iranian revolutionary guards, to Baghdad to discuss the military approach. Iran, the great Shia power, is determined not to see Shia-majority Iraq fall to the Sunni milita.
US president Barack Obama is weighing up what help to give Baghdad to counter Isis, which has taken control of major cities in the north.
The Pentagon has sent an aircraft carrier to the Gulf in advance of potential air strikes amid calls for Obama to talk with Iran over a co-ordinated response.
Former prime minister Tony Blair took to the media yesterday to make the case for a tough response to the extremist insurgency - arguing that it was caused by a failure to deal with the Syria crisis, not the invasion by US and British forces 11 years ago.
His remarks sparked a furious reaction from London mayor Boris Johnson, who accused the ex-Labour leader of being "unhinged" and having sent UK forces into the bloody conflict in part to gain personal "grandeur".