Iraqi authorities have begun excavating the site of a mass grave believed to contain thousands of victims of the Islamic State (IS) group near Mosul city, the project's director told AFP on Sunday.

The first phase, which was launched on August 10, includes surface-level excavation at the Khasfa site, director Ahmed al-Assadi said.

An AFP correspondent visiting the site in northern Iraq on Sunday said the team unearthed human skulls buried in the sand.

Khasfa is located near Mosul, where IS had established the capital of their self-declared "caliphate" before being defeated in Iraq in late 2017.

Assadi said that there were no precise figures for the numbers of victims buried there -- one of dozens of mass graves IS left behind in Iraq -- but a UN report from 2018 said Khasfa was likely the country's largest.

Official estimates put the number of bodies buried at the site at at least 4,000, with the possibility of thousands more.

The project director said the victims buried there include "soldiers executed by IS", members of the Yazidi minority and residents of Mosul.

Exhuming the bodies from Khasfa is particularly difficult, Assadi said, as underground sulphur water makes the earth very porous.

The water may have also eroded the human remains, complicating DNA identification of victims, he added.

Assadi said further studies will be required before his team can dig deeper and exhume bodies at the site -- a sinkhole about 150-metre (nearly 500-foot) deep and 110-metre wide.

Iraqi authorities said it was the site of "one of the worst massacres" committed by IS jihadists, executing 280 in a single day in 2016, many of them interior ministry employees.

In a lightning advance that began in 2014, IS had seized large swathes Iraq and neighbouring Syria, enforcing a strict interpretation of Islamic law and committing widespread abuses.

The United Nations estimates the jihadists left behind more than 200 mass graves which might contain as many as 12,000 bodies.

In addition to IS-era mass graves, Iraqi authorities continue to unearth such sites dating to the rule of Saddam Hussein, who was toppled in a US-led invasion in 2003.

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