(CNN) Mass graves containing the remains of civilians executed by ISIS have been found in the disputed Iraqi province of Kirkuk, Iraqi authorities told reporters on Saturday.

The news conference was held at the site of the mass graves, which were discovered in the al-Bakara area, about 3 kilometers north of the city of Hawija. That area was an American base prior to 2011, Kirkuk Governor Rakan Saeed said.

"We are standing here, where ... at least 400 civilians were dragged, some in their red jumpsuits, and brutally executed by ISIS," he said.

Colonel Murtada Abbas of the 60th Brigade in the Iraqi military said witnesses to the executions led them to the graves.

Saad Abbas al-Nuaimi, a shepherd in the area, told CNN he saw members of ISIS drive vehicles loaded with civilians to the site and execute them. He said there are five mass graves in the al-Bakara area.

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