Karim Khan on Monday told the Security Council the team also concluded war crimes were committed by the Islamic State group against predominantly Shiite unarmed cadets and personnel from the Tikrit Air Academy who were captured, tortured and subjected to mass execution in June 2014.

He said an Islamic State video released in July 2015 showing the killings “constitutes a direct and public incitement to commit genocide against Shia Muslims”.

The Security Council voted unanimously in September 2017 to ask the UN to establish an investigative team to help Iraq preserve evidence and promote accountability for what “may amount to war crimes, crimes against humanity and genocide” committed by Islamic State extremists, both in Iraq and the Levant, which includes Syria.

In his sixth report to the council, Khan said the UN Investigative Team to Promote Accountability for Crimes committed by the Islamic State group, also known as Daesh, ISIL and ISIS, rapidly expanded the amount of evidence it has in the last six months.

He said “significant developments” in collecting forensic evidence from mass grave sites, digital data extracted from hard drives that belonged to the IS group, digitisation of case files, and use of advanced technological tools to process and search databases has allowed the team “to establish clear timelines of activities of key ISIL members.” Khan called it “a landmark moment” that the team, known as UNITAD, had established convincing evidence that Islamic State extremists committed genocide “against the Yazidi as a religious group'' with the intent “to destroy the Yazidi physically and biologically”.

This was manifest in the IS ultimatum applied to all Yazidis “to convert or die” and led to thousands killed, “either executed en masse, shot as they fled, or dying from exposure on Mount Sinjar as they tried to escape,” Khan said.

“Thousands more were enslaved, with women and children abducted from their families and subjected to the most brutal abuses, including serial rape and other forms of unendurable sexual violence” that for many lasted years, “often leading to death”.

Khan added that crimes against the Yazidis continue, with thousands of women and children separated from their families or missing and some still with their their IS captors or those to whom they were sold.

In 2016, the UN-mandated Independent International Commission of Inquiry on Syria said the Islamic State group was committing genocide against Yazidis, and several non-governmental organisations have echoed that conclusion.

But Khan said what UNITAD has done in terms of the Yazidis is more important because the team was mandated to look at a variety of evidence that could stand up in court where the burden of proof is on the prosecution -- “and to not just draw brush strokes from a survey of victims.” He said information from electronic devices that belonged to IS extremists also led UNITAD to open a new investigation “into the development and successful deployment of chemical and biological weapons by ISIL in Iraq”.

Evidence collected by UNITAD details how the militant group used laboratories at Mosul University “as the epicentre of its chemical weapons program, drawing on the expertise of scientists and medical professionals from Iraq and abroad,” Khan said.

Initially, he said, IS weaponised chlorine from water treatment plants captured by its fighters in 2014, and subsequently developed “toxic lethal compounds including thallium and nicotine that were tested on live prisoners, leading to death”.

IS then developed a system to produce mustard gas, also called sulfur mustard, “that was deployed in March 2016 through the firing of 40 rockets at the Turkmen Shia town of Taza Khurmatu,” Khan said.

Khan, who will become chief prosecutor at the International Criminal Court on June 15, said this investigation is rapidly progressing, with initial results anticipated to be completed within five months.

By the end of the year, he said, the team also anticipates initial results “addressing crimes against minority Christians, Kaka'i, Shabak, Shia Turkmen and Sunni communities in Iraq, as well as the massacre of predominantly Shia inmates at Badush prison”.

Khan said the next step is to use the information and evidence collected by UNITAD “to meet the expectations of survivors” and put it before national courts to prosecute those responsible for these “horrific crimes”.

Nobel Peace Prize laureate Nadia Murad, a Yazidi forced into sexual slavery by IS fighters who killed her mother and six brothers, urged the Security Council to refer the genocide against her people to the International Criminal Court or establish a court to prosecute those responsible for the atrocities.

“I ask you to start a new chapter -- legal accountability for ISIS crimes would dramatically impact every action of my community's recovery,” she said. “It is time for the international community to do, more than listen. It is time to act. If world leaders have the political will to act on this evidence, then justice is truly within reach.”