Forensic teams have begun excavating suspected mass graves in Tikrit believed to contain up to 1,700 bodies, just days after Iraqi forces re-took the city from the clutches of Isis militants.

The 12 sites are believed to hold up to 1,700 Iraqi soldiers, who were brutally killed in a mass execution that was photographed and circulated online by Isis as propaganda.

The soldiers were reportedly from Camp Speicher, a former US base outside of Tikrit, the home town of Saddam Hussein. The graves confirm findings by Human Rights Watch (HRW), who analysed photographs released by jihadists and satellite images showing mass graves shortly after the extremist group swept through northern Iraq in its bloody insurgency.

The victims are believed to be mostly Shia soldiers and the massacre was denounced as one of the worst since Isis began its rapid advance across the country.

"We dug up the first mass grave site today. Until now we found at least 20 bodies. Initial indications show indisputably that they were from the Speicher victims," Khalid al-Atbi, an Iraqi health official working with the forensic team sent to Tikrit told Reuters.

Remnants of a body belonging to Shia soldiers from Camp Speicher (Reuters)

"It was a heartbreaking scene. We couldn't prevent ourselves from breaking down in tears. What savage barbarian could kill 1,700 persons in cold blood?"

One survivor told the news agency he was covered in the blood of his fellow soldiers and only escaped execution by pretending to be dead.

A hand is seen as Iraqi forensic teams recovered dead bodies from a mass grave (Reuters)

"The fourth bullet was fired, but I wasn't hit. The fifth bullet killed the man next to me. Playing dead, I fell to the ground. I was covered by the blood of my colleagues and I rolled down into the valley," he said.

In pictures: Fight for Tikrit Show all 16 1 /16 In pictures: Fight for Tikrit In pictures: Fight for Tikrit Fight for Tikrit Shiite fighters pose in front of a mural depicting the emblem of the Islamic State group outside one of the presidential palaces in Tikrit, on April 1, 2015 Getty Images In pictures: Fight for Tikrit Fight for Tikrit Iraqi Prime Minister Haider al-Abadi tours the city of Tikrit after it was retaken by the security forces in Baghdad, Iraq, on April 1 2015 AP In pictures: Fight for Tikrit Fight for Tikrit An Iraqi flag waves over the Tigris River om April 1 2015 AP In pictures: Fight for Tikrit Fight for Tikrit Iraqi security forces playing football in Tikrit after their country declared a 'magnificent victory' over the Islamic State group in the city on April 1 2015 Reuters In pictures: Fight for Tikrit Fight for Tikrit A member of the security forces holds Iraq’s national flag in front of a painted Isis emblem Getty Images In pictures: Fight for Tikrit Fight for Tikrit Iraqi security forces arrest a member of the Islamic State in Tikrit on April 1, 2015 Reuters In pictures: Fight for Tikrit Fight for Tikrit A member from the Iraqi security forces beats an Isis insurgent, who was captured in Tikrit Reuters In pictures: Fight for Tikrit Fight for Tikrit Shiite fighters from the Popular Mobilisation units celebrate in front of the the provincial council building inside the northern Iraqi city of Tikrit on March 31, 2015 Getty Images In pictures: Fight for Tikrit Fight for Tikrit Iraqi soldiers stand on a damaged tank during fighting against IS militants near Tikrit Corbis In pictures: Fight for Tikrit Fight for Tikrit Fighters of Iraqi Shiite militias who volunteered to support the Iraqi forces make their way towards Daquq town near Tikrit Corbis In pictures: Fight for Tikrit Fight for Tikrit Iraqi security forces and Shia fighters chant slogans as they gather at Udhaim dam, north of Baghdad, where they have been massing in preparation for an attack on Isis along the Tigris River Reuters In pictures: Fight for Tikrit Fight for Tikrit Pro-government volunteer Shia fighters in Awaynat near Tikrit AFP/Getty In pictures: Fight for Tikrit Fight for Tikrit Iraqi government forces and allied militias firing weaponry from a position in the northern part of Diyala AFP In pictures: Fight for Tikrit Fight for Tikrit A volunteer in the Iraqi Shiite Badr Army militia taking up position outside Tikrit Corbis In pictures: Fight for Tikrit Fight for Tikrit Volunteers of the Iraqi Shiite Badr Army militia patrol an area at eastern Tikrit city Corbis In pictures: Fight for Tikrit Fight for Tikrit Members of the Iraqi security forces coming from the city of Samarra drive towards al-Dawr south of Tikrit to launch an assault against the Islamic State AFP/Getty

An unverified statement from Isis militants on Twitter claimed to have executed 1,700 prisoners, before it posted about 60 gruesome pictures of groups of men being forced to lie in a shallow trench with their arms tied.

A picture posted on a militant website reportedly shows jihadists from Isis taking aim at captured Iraqi soldiers wearing plain clothes, after taking over a base in Tikrit (AP)