Iraqi investigators carried out an initial examination on Tuesday of a mass grave site discovered in an area south of Mosul that was recently retaken from the Islamic State group.

Iraqi security forces announced the discovery of the site in the Hamam al-Alil area the day before, after retaking it as part of the operation to recapture Mosul, the last IS-held Iraqi city.

A dirt barrier borders the site, where body parts and bones are visible among rubbish that has been dumped there.

Men in Iraqi security forces uniforms used ropes to pull two bodies, one of them headless, from the grave, and also removed a decapitated head, but they were later told to return them to their original locations.

The investigators, some of whom wore face masks because of the smell, took notes at the grave site.

"Today, the team conducted an initial examination," said Mohammed Taher al-Tamimi, an official from an Iraqi cabinet office that he said is coordinating and supporting efforts to investigate the site.

Tamimi described the killings at the site as a "massacre", and said the victims had been blindfolded and had their hands and feet bound.

Some bodies were missing their heads, while those of others had been broken into pieces, he said.

Iraq's Joint Operations Command said on Monday that 100 headless bodies had been found in the Hamam al-Alil area -- an assertion contradicted by Tamimi.

- Sound of gunshots -

"From what we saw today, I believe that there are around 25 bodies visible. But this does not mean that this is the total number. We believe that there are very large numbers there," Tamimi said.

Dhiyab Tareq, a 32-year-old from the area, said he had heard shots when IS carried out executions at the site.

"I was sitting close to the door and heard the gunshots," Tareq said, adding that the following day IS members boasted about killing members of the security forces.

Captain Mahmud Ayil, a federal police media official, said the grave site was discovered after people provided information about its location.

"The units entered and found that they (IS) had rigged its outskirts" with explosives, Ayil said.

Another police officer said that the way into the site had been cleared of bombs but that there may be more on the edges of the site.

Another bomb-rigged mass grave was discovered in the Sinjar area west of Mosul last year.

Bombs are a key part of both offensive and defensive operations by IS, and pose a danger even after places are recaptured from the jihadists.

IS overran large areas north and west of Baghdad in 2014, declaring a cross-border "caliphate" that also included territory in Syria.

Its rule has been marked by repeated atrocities including mass beheadings and other executions that it has documented in photos and videos lauding the violence that its supporters share online.

Iraqi forces have since regained much of the territory that IS seized, and have uncovered a series of mass graves and massacre sites as they have pushed the jihadists back.