Sickening pictures and videos have emerged that claim to show Iraqi soldiers torturing and beheading Islamic State prisoners in apparent revenge attacks. The images have been posted on social media in recent days reportedly by U.S.-trained Iraqi Special Forces who are advancing against jihadi strongholds. One Instagram account under the name of Iraqi_lion18 contains numerous pictures of severed heads and mutilated bodies. The account holder vowed 'revenge' for the massacre by ISIS of hundreds of military cadets at a base near Tikrit called Camp Speicher in June. Brutality: This picture posted on social media claims to show an Islamic State prisoner being thrown to his death from a tower by Iraqi soldiers who are advancing on the terror group's strongholds in Iraq . Savage: Men believed to be Iraqi Special Operations Forces from the 'Golden Brigades' surround an  ISIS suspect who has been pulled by a rope . Horrific: A soldier stands on two severed heads in what is being reported as a revenge attack on ISIS fighters . The horrific cache of images, uncovered by ABC News, emerged after Shi'ite militia leaders fighting with Iraqi troops said the current offensive on Saddam Hussein's hometown would be a chance to avenge their deaths. Other accounts show a man with his hands bound being thrown out of a tower and men believed to be Iraqi Special Operations Forces from the 'Golden Brigades' surrounding an alleged ISIS suspect who has been pulled by a rope. MailOnline has not been able to independently verify the origin of the images or establish when they were taken. But they have been shown to senior military figures, human rights experts and politicians who called them evidence of Iraqi 'war crimes.' Retired U.S. Army Special Forces Lt Col James Gavrilis also told ABC News that the images 'did not look like believe  ISIS propaganda at all'. Allegations: Images like this have been shown to senior military figures, human rights experts and politicians who called them evidence of Iraqi 'war crimes'. An investigation has been launched by the Iraqi military . Ex-US Army Special Forces Lt Col James Gavrilis said he did not believe the images were ISIS propaganda . U.S.officials confirmed yesterday that Washington was withholding funding from some Iraqi units 'on the basis of credible information in the past' of atrocities. After being shown the images, Iraqi Army spokesperson, General Saad Maan, said an investigation would be launched, adding: 'We don't have anything to hide.' It came as a prominent Iraqi Sunni preacher today urged authorities to prevent Shi'ite militias fighting alongside government forces against ISIS in Tikrit from carrying out revenge attacks. In his appeal, Sheik Abdel Sattar Abdul Jabbar cited reports of Shi'ite militiamen burning Sunni homes in the battle to win back Tikrit from Islamic State hands. Abdul Jabbar said that if the government failed to stop revenge attacks, the country would face reignited sectarian tensions - such as the ones witnessed at the height of the war in 2006 and 2007. Gunned down: A man believed to be an ISIS militant is shot  after reportedly being captured by Iraqi forces . 'We ask that actions follow words to punish those who are attacking houses in Tikrit,' Abdul Jabbar said during his Friday sermon in Baghdad. 'We are sorry about those acting in revenge that might ignite tribal anger and add to our sectarian problems.' Iraqi forces entered Tikrit for the first time on Wednesday from the north and south. On Friday, they fought fierce battles to secure the northern Tikrit neighborhood of Qadisiyya and lobbed mortars and rockets into the city center, still in the hands of ISIS. Iraqi military officials have said they expect to reach the centre of Tikrit within two to three days. Condemned: A man is pictured being dragged along by what appears to be a military vehicle . 'Tortured': U.S.officials confirmed yesterday that Washington was withholding funding from some Iraqi units 'on the basis of credible information in the past' of atrocities . Meanwhile, Iraq's top Shi'ite cleric called on the government to step up support for Shiite militias battling ISIS alongside Iraqi troops and to take care of fallen militiamen's families. 'The government should provide care for those who are defending the country with their blood and pay attention to the families of the martyrs, those families have lost their dear one and sons,' Grand Ayatollah Ali al-Sistani said. His remarks were relayed by his spokesman Ahmed al-Safi in the Shi'ite holy city of Karbala. 'The excuses given for the delays in paying the salaries of the fighters are unacceptable.' The Iranian-backed Shi'ite militias have played a crucial role in regaining territory from the Islamic State group, supporting Iraq's embattled military and police forces in battle. Mourners carry the coffin of a Shi'ite fighter who was killed in clashes with Islamic State militants in Tikrit . Struck down: A Shi'ite fighter injured during clashes with Islamic State militants in northern Tikrit . U.S. General Martin Dempsey said Wednesday that as many as 20,000 militiamen may be involved in the ongoing Tikrit operations. Prime Minister Haider al-Abadi last week called on his forces to protect civilians and their property in areas reclaimed from ISIS, vowing zero tolerance for any violations. He also called upon Sunnis who may have welcomed the initial onslaught by the Islamic State or even took up arms on the side of the militants. 'I call upon those who have been misled or committed a mistake to lay down arms and join their people and security forces in order to liberate their cities,' al-Abadi said. Amnesty International cautioned it was still too early to tell if Tikrit had been a cleaner operation than usual. 'For now access to combat areas is restricted and information about abuses may take time to filter out,' the rights group's senior crisis response adviser Donatella Rovera told AFP.
WARNING GRAPHIC CONTENT . Pictures were posted on social media 'by U.S.-trained Iraqi Special Forces'
One account shows numerous images of beheadings and mutilated bodies . Account holder vows 'revenge' for massacre of hundreds of cadets by ISIS . Shi'ite militia claimed offensive on Tikrit would be opportunity for revenge . Images shown to U.S. leaders who say they are evidence of 'war crimes'
Iraqi Army spokesman says investigation will be launched into claims .