A satellite imagery group says it has new evidence of mass graves in Sudan's Southern Kordofan state, where the government is accused of committing atrocities against civilians.

In a report released Wednesday, the U.S.-based Satellite Sentinel Project says images from space reveal two new mass graves, bringing to eight the total number it has discovered in Southern Kordofan.

The group accuses the Sudanese Red Crescent Society of being involved in digging the graves, filling them with corpses and in some cases pouring fuel on the bodies before setting them on fire. The satellite imagery group cites evidence that includes witness reports and Red Crescent statements, in addition to satellite pictures.

Sudan's government is engaged in heavy fighting with ethnic Nuba rebels, who are seen as supporters of South Sudan, which declared independence from the north in July.

Sudanese officials deny the claims in the report, saying there is no proof of mass graves in the area. They blame rebels for the deaths and accuse them of spreading false reports.

The United Nations said Wednesday it could not confirm the report. A spokeswoman said the U.N. no longer has a mandate in the region so it cannot verify the information.

The U.N., however, says it has received reports of indiscriminate killings, widespread looting and massive civilian displacement in Southern Kordofan. It blames most of the violence on Sudan's army, police and allied militia.

On Tuesday, Sudanese President Omar al-Bashir called for a two-week, unilateral cease-fire in Southern Kordofan. Speaking in the state capital, Kadugli, President Bashir said the situation on the ground would be assessed after the truce.

The fighting near the Sudan-South Sudan border has forced tens of thousands of Nuba from their homes. Nuba fighters sided with the south during Sudan's 21-year north-south civil war.