Kennedy also endorsed an investigation against Syria and Iraq for failing to prevent the killings.

The groundbreaking report, compiled by a group of prominent human rights lawyers, seeks to highlight states' binding responsibility to prevent genocide on their territories, even if perpetrated by a third party such as extremist organizations.

The lawyers, known as the Yazidi Justice Committee (YJC), asserted that states are held accountable under the Genocide Convention to prevent genocide.

Sir Geoffrey Nice QC, chair of the YJC, described the genocide of the Yazidi people as “madness heaped on evil”.

“Mechanisms in place could have saved the Yazidis from what is now part of their past, and part of their past partial destruction,” he said.

From 2013, a genocide against the Yazidis, a religious minority in Iraq and Syria, has been attempted. Following a three-year investigation into the conduct of 13 countries, the 278-page report concluded that three of them failed in their duty to take reasonable steps to prevent genocide.

Regarding Turkey, the YJC accused its leaders of being complicit in the massacres, claiming that the country failed to police its borders to prevent the free flow of extremist fighters, including a significant number of Turkish nationals. Turkish officials have dismissed the criticisms as unfounded.

The YJC also claimed that from April 2014, Turkish officials turned a blind eye to the sale, transfer and enslavement of Yazidi women and children,and assisted in training fighters affiliated with extremist organizations to fight its Kurdish enemies in Syria, thus strengthening the perpetrators of the genocide.

“Turkish officials knew and/or were willfully blind to evidence that these individuals would use this training to commit prohibited acts against the Yazidis,” the report said.

Although the report acknowledged that Iraq had called on the UN to recognize the atrocities committed in 2014, it accused the Iraqi government of failing to coordinate with Kurdish authorities or take steps to evacuate the Yazidis to safety.

According to the report, the Syrian government also failed to prevent the transfer and detention of enslaved Yazidis on its territory.

The Turkish ambassador to the UK, Ümit Yalçın, called the criticisms baseless and unfair.

“Turkey starting from the early years of the conflict in Syria played a key role in the protection of Syrian civilians and minorities, including Yazidis, in the region against the attacks and violations of terrorist groups,” Yalçın said.

He also added: “Turkey not only opened its doors and became a safe haven for millions of Syrians and Yazidis but also provided protection for the people of the region through three counter terrorism operations in Syria. Today Yazidis live peacefully in areas that are under the control of the legitimate Syrian opposition in north-western Syria.

“Moreover, last year many Yazidi families that took refugee in north-western Syria tried to return to their homes in Syria’s north-east but [were] prevented from doing so by PKK/YPG [the initials of the Kurdish groups in Turkey and Syria].”

“An ocean of impunity exists in relation to the Yazidi genocide”, Kennedy said, noting that extremist groups as a non-state actors cannot be prosecuted under international law.

Kennedy added that meanwhile, states had “failed to in their duty to address their responsibilities to prevent the genocide for a variety of inhumane reasons”. She wrote that if they are not held accountable, “then the promise of ‘never again’ rings hollow”.