Baghdad - The Sunni extremist group that has taken territory across Iraq has posted photos online that appear to show its fighters massacring Iraqi soldiers, BBC news has reported.The army personnel are being led away and then lying in trenches before and after their “execution”. Iraqi military spokesman Lt Gen Qassim al-Moussawi said the pictures were authentic and depicted events in Salahuddin province. But the authenticity has not been independently confirmed. The BBC’s Jim Muir, in northern Iraq, says if the photographs are genuine, it would be by far the biggest single atrocity since the time of the American-led invasion of Iraq in 2003. Their emergence came as the Iraqi government claimed to have “regained the initiative” against the offensive by Sunni rebels led by ISIS - the Islamic State in Iraq and the Levant. Extremists captured key cities, including Mosul and Tikrit, last week, but several towns have now been retaken from the rebels. The pictures, apparently posted by ISIS, are said to show what happened to soldiers after the group took over an army base in Tikrit following the surrender of the garrison there. Video footage, apparently filmed earlier, shows many hundreds of men being marched off from near the Speicher base, with the voice on tape saying they were soldiers who had surrendered at the facility. Still photographs show large numbers of young men being transported away in trucks.Captions on the photographs, which again appeared to have been issued by ISIS, said they were being taken away to their deaths. What appear to be ISIS fighters are seen firing on their captives in some images.