A U.S.-based satellite imagery organization says it has found new evidence of mass graves in Sudan's oil-rich Southern Kordofan state.

On Wednesday, the Satellite Sentinel Project released eyewitness reports and images of what it says are machinery and body bags used by the Sudanese Red Crescent Society to excavate and fill the burial sites.

The organization says it has now identified a total of eight mass graves in the region, including the two announced today.

Sudan's government rejects the group's findings. The government blames violence in the region on rebel groups and the army of newly independent South Sudan.

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

Fighting between government forces and Southern Kordofan's ethnic Nuba rebels broke out in June, about a month before South Sudan split from the north and declared independence. The fighting near the Sudan-South Sudan border drove tens of thousands of Nuba from their homes.

Nuba fighters sided with the south during Sudan's 21-year civil war.