A key Iraqi diplomat yesterday accused Islamic State militants of harvesting and trafficking organs to help finance its brutal operations in the Middle East.

Iraq's ambassador to the United Nations, Mohamed Alhakim, urged the Security Council to investigate. He said mass graves have been uncovered in which bodies are missing organs and bear signs of surgical incisions.

"Some of the bodies we found are mutilated...that means some parts are missing," he told reporters. "This is clearly something bigger than we think."

Alhakim also claimed that 12 doctors have been executed by IS in Mosul after refusing to help remove organs, and said militants are participating in "crimes of genocide".

It is not the first time allegations of organ trafficking have been levelled at the Islamist group. A report by the al-Monitor news website in December claimed that IS has established a specialist organ-smuggling faction within its ranks.

According to an Iraqi doctor named in that report, organs are being taken from dead militants, injured people and those who have been kidnapped. They are then allegedly smuggled out of Syria and Iraq into neighbouring countries, and sold across the world.

The outgoing UN envoy to Iraq, Nikolay Mladenov, yesterday responded to Alhakim's allegations by saying "it's very clear that the tactics [IS] is using expand by the day." He confirmed that the UN would look into the claims, but told reporters that they could not yet be confirmed.

"At this point we're not in a position to corroborate what he says, but obviously any source of illegal financing of groups such as ISIS or other extremist groups is extremely worrisome," UN spokesman Stephane Dujarric told CNN.

The latest claims about the terrorist group follow several incidents of brutality perpetuated by IS militants. On Sunday, a video was released showing the beheading of 21 Egyptian Christian men kidnapped in Libya.

Insurgents also allegedly burned to death 45 people, thought to be members of the Iraqi security forces, in the western town of al-Baghdadi earlier this week. The town in Anbar province had been besieged by IS since October, and finally fell to the jihadists on January 12.