As the battle for control of Iraq continues, more civilians fall victim to war. With the newly created Islamic State moving closed to Baghdad, Iraqi government troops and Shiite militants have committed atrocities against Sunnis as they retreat.

Around 255 men and boys have been killed by Iraqi government forces or Shiite militants associated with the government, according to the AFP. The discovery of the mass killings was made by Human Rights Watch and is explained in a lengthy report.

Those killed are all believed to be Sunnis, the same Muslim sect as the Islamic State (IS), formerly known as the Islamic State of Iraq and the Levant (ISIL). The Sunni extremist group has advanced through Iraq in the last couple of months, capturing various Iraqi cities, including Iraq's second largest city of Mosul. Since then, according to AFP, it has renamed itself as a "caliphate" spanning their territory on Iraq and Syria.

Iraqi officials speaking with BBC News denied the claims that government troops killed Sunnis. Instead, Lt. Gen. Qassim Atta, a military spokesman for the Iraqi government, said the men died "as a result of terrorist acts."

However, the Human Rights Watch disagrees. The thorough report compiled from various sources, including eyewitnesses, government officials and other news reports, conclude that Iraqi government forces and Shiite militants have killed hundreds.

The report gathered information on five massacres from June 9-21 in five different cities, including Mosul, Tal Afar, Baaquba, Jumarkhe and Rawa. All the cities are either in conflict areas or under the control of IS now. However, the report found that as Iraqi troops retreated, they began massacring Sunni prisoners in, what HRW believed to be, revenge or to prevent them from joining IS.

"Gunning down prisoners is an outrageous violation of international law," said Joe Stork, deputy Middle East director at Human Rights Watch. "While the world rightly denounces the atrocious acts of ISIS, it should not turn a blind eye to sectarian killing sprees by government and pro-government forces."

HRW explained that many of the men were killed execution style, with corroborating accounts by witnesses, including medical personnel in one case. In the city of Mosul, guards threw a grenade into a cell holding prisoners, killing six. The high level of sectarian violence has led to calls for further investigation.

"An international commission of inquiry or a similar mechanism should investigate serious violations of the laws of war and international human rights law by all sides in the Iraq conflict, including by government forces, pro-government militias, and ISIS and associated forces," a statement by HRW said.

"The inquiry should be mandated to establish the facts, and identify those responsible for serious violations with a view to ensuring that they are held accountable. The inquiry should collect and conserve information related to abuses for future use by judicial institutions."