Satellite images show mass graves following recent fighting in Sudan, a campaign group set up by Hollywood star George Clooney says.
The Satellite Sentinel project says the apparent massacre took place in the town of Kadugli, in South Kordofan.
Fighting between rebels from the Nuba Mountains and the Sudanese armed forces began in the state last month. Despite rights groups denouncing the brutal killing of the Nuba people, the Sudanese government has until now denied the allegations that civilians had been killed and insist it is fighting a legitimate war of self-defence against insurgents.
South Kordofan borders South Sudan, the latest African nation to be born, and despite Omar al-Bashir's denial, the conflict was so intense that an estimated 70,000 people have fled the recent fighting there.
The Satellite Sentinel project says the satellite images, which appear to show mass graves are consistent with allegations the Sudanese armed forces and government-aligned militias have targeted civilians and the organisation says it has spoken with a number of witnesses that consistently talk about the systematic killing of civilians thought to be part of the opposition by the Bashir government's forces.
Many residents of the Nuba Mountains fought with southern rebels during the two decade north-south war but now find themselves in the north and thus under the north authority.
A recent UN report also said bombing and fighting was continuing in South Kordofan despite the peace deal, but with journalists and diplomats barred from entering the region, obtaining reliable and accurate information is particularly difficult.
The organisation also says it has spoken to a number of witnesses who allege forces aligned with Khartoum are systematically killing civilians believed to be opposed to the government.
Mr Bashir has recognised South Sudan and while he last week praised the new country, he warned that peace with Sudan's new neighbour was conditional on mutual respect of borders and non-interference in each other's affairs.
The two sides agreed last month, in a deal brokered by the African Union, to integrate the Nuba fighters into the national army or disarm them voluntarily, but Bashir announced recently that he was not willing to negotiate with what he called the insurgents.
Meanwhile, an official in Khartoum's ruling party, Qutbi al-Mahdi, has accused aid agencies of giving logistical support to the rebels, the pro-government Sudanese Media Centre (SMC) reports.
He warned the agencies that they risked "legal penalties" and expulsion, SMC said.
In 2009, Sudan expelled 10 humanitarian organisations from the western region Darfur, accusing them of collaborating with the International Criminal Court (ICC), which issued an arrest warrant for Mr Bashir for alleged genocide and crimes against humanity during Darfur's eight-year conflict.
There is also an arrest warrant for Ahmed Haroun, a former Darfur governor who is now South Kordofan's governor.