It's the first time an official has estimated the total number of Yazidis found in grave sites littered around Sinjar since Iraqi forces pushed the terror group out more than two years ago.
Thirty-five mass graves along with 100 individual graves have been unearthed, according to Hussein Hassoun, the spokesman of the Higher Committee to Introduce Yazidi Kurds.
"The mass graves are about 5 to 10 kilometers apart," Hassoun told CNN. Between 300 and 500 bodies were found in the past few months.
A new mass grave was recently found in the Hardan area of Sinjar, the town's mayor told Rudaw news agency.
Most of the bodies have been left in their graves, so officials can only give an estimate of the total number of dead discovered thus far. Due to limited resources, only 65 bodies have been exhumed, Fouad Othman, the spokesman of the Martyrs Ministry in Kurdistan, told CNN.
ISIS swept into the Sinjar area in August 2014
, displacing, killing and enslaving thousands of Yazidis -- an ancient ethnic and religious minority who are descendants of Kurds and call modern-day Iraq home.
ISIS was driven out of Sinjar in 2015, and Kurdish officials believe more mass graves will be found as more territory is seized from the terror group.
A US-led coalition has mounted an aggressive campaign to recapture ISIS-held territory in recent months. ISIS currently controls less than 7% of Iraqi territory, Iraq's military says -- down from nearly 40% of the country in 2014.
Hassoun said intelligence reports indicate that there could be up to 12 mass graves in the village of Kocho, which remains under ISIS control.
The village is among two identified by the UN report as sites of large mass killings, which the report defines as more than 20 men and boys killed at the same time.
Hassoun told CNN that his committee lacked the funds to conduct DNA tests needed to match the dead with family members looking for their loved ones' remains.
He said more international support would be needed to determine the true scale of ISIS' atrocities against Yazidis.