Iraqi authorities have uncovered a mass grave in Ramadi containing at least 40 bodies, including women and children, apparently killed by Islamic State insurgents when they seized the city in May, police and provincial officials said.

Footage posted on the Facebook page of the provincial police on Wednesday showed what appeared to be bodies in varying states of decay being pulled from a shallow grave in the capital of Anbar province which Iraq's military recaptured last month.

Police chief Major General Hadi Razij spoke in the video about the grave, and an adviser to the governor confirmed the images were authentic. Interior Ministry spokesman Brigadier General Saad Maan confirmed the reports.

"We believe they were the last to fight #DAESH before #Ramadi fell in May 2015. Investigation ongoing.", Anbar Governor Sohaib al-Rawi said in a tweet with a picture of body bags lying in a street. Daesh is the Arabic acronym for Islamic State, which is also called ISIS or ISIL.

Islamic State overran Ramadi last year as the Iraqi army abandoned its posts for the second time in less than a year, setting back government efforts to push back the ultra-hardline Sunni militants.

The military, backed by US-led coalition air strikes, recaptured the city in December, but widespread destruction and explosives planted by the insurgents in streets and houses have prevented civilians from returning.

Several mass graves have been uncovered in areas retaken from Islamic State, which imposes strict restrictions and harsh punishments on the millions of civilians living under its control.

The United Nations has said the militants are responsible for acts that may amount to war crimes, crimes against humanity and possibly genocide.

Muhannad Haimour, the governor's adviser, told Reuters at least 15 of the bodies, which were discovered in the central district of al-Jamaaiya, belonged to police officers, according to ID cards retrieved from the grave.

He said not all the bodies had been identified, but some were believed to belong to women and children related to the police.

Haimour said the grave was discovered through interrogations with captured Islamic State militants.

It was not clear how the victims were killed, but the video appeared to show some of them had been handcuffed at the time of death. Haimour said there were signs of torture and gunshot wounds on some bodies, but that could not be immediately confirmed.

Ramadi is the largest city retaken from Islamic State since the insurgents swept across large swathes of Iraq and neighboring Syria in mid-2014, declaring a modern-day caliphate and killing or capturing thousands of people.