Iraqi police on Tuesday unearthed two mass graves in the western city of Ramadi, with bodies of about 40 people killed by Islamic State militants during the militant Sunni group's reign of terror in the city, officials said.

The officials said IS militants who were captured and arrested after Iraqi forces routed the extremists from the Anbar provincial capital led authorities to the site of the mass graves, inside the city's soccer stadium.

Bodies of women and children were among those found in the two graves, along with bodies of men in civilian clothes, said the officials, speaking on condition of anonymity because they were not authorized to talk to reporters.

Ramadi, the provincial capital of Anbar, fell to IS in May, a major setback for U.S.-allied Iraqi forces at the time. It was liberated in December.

There have been many instances of mass graves being uncovered in territory wrested from IS militants in Iraq and Syria — thousands of people have been killed in summary and extrajudicial killings by the Sunni militants and the graves have been a dark testimony to the group's brutality.

In June 2014, some 1,700 Iraqi soldiers were captured and then killed by IS militants when they overran Saddam Hussein's hometown of Tikrit. At the time, the soldiers were trying to flee from Camp Speicher, a nearby army base where they were deployed. Mass graves with hundreds of Iraqi soldiers' bodies were found after the city was liberated in April 2015.

In December, the U.N. human rights office in Iraq said it received reports of 16 mass graves discovered near the town of Sinjar after it was liberated from the Islamic State group the previous month.

Among the first mass graves uncovered in Sinjar — within days of IS forces being pushed out of the town — was one near the town's center that has been estimated to contain the bodies of 78 elderly women, and another, about 15 kilometers (10 miles) outside of Sinjar, with between 50 and 60 bodies of men, women and children.

The U.N. uses the term mass grave to refer to a location where three or more victims of what the world body defines as extra-judicial, summary or arbitrary executions are buried — not those who have been killed in combat, attacks such as bombings or armed confrontation.