Satellite images show mass graves following recent fighting in Sudan, a campaign group set up by Hollywood star George Clooney says.
Eyewitnesses who spoke to the group said soldiers had systematically massacred civilians in South Kordofan.
Fighting began in the state last month, between rebels from the Nuba mountains and the Sudanese armed forces.
A Sudanese military spokesman denied allegations that civilians had been killed.
South Kordofan borders South Sudan, which last week became an independent state.
Some 70,000 people have fled the recent fighting there.
The Satellite Sentinel project says the satellite images show three apparent mass graves.
An eyewitness told the group that about 100 bodies were taken to a site near a school in the village of Tilo, not far from the state capital, Kadugli, after fighting last month.
"The DigitalGlobe satellite images contain many of the details and hallmarks of the mass atrocities described by at least five eyewitnesses to the alleged killings," said Nathaniel A Raymond, of the Harvard Humanitarian Initiative, which analyses the project's images.
Although a peace deal was signed recently, a UN report this week said bombing and fighting was continuing in South Kordofan.
Sudan's ruling National Congress Party spokesman Rabie A Atti strongly denied the claims, saying they were designed to discredit the government.
"People can go there and visit the area and see what is the actual reality," the AP news agency quotes him as saying.
The BBC's James Copnall in Sudan's capital, Khartoum, reports that it is extremely difficult to get accurate information, as journalists and diplomats are barred from the region and the UN faces restrictions on its movement.
The Satellite Sentinel project says the apparent massacre took place in the town of Kadugli, in South Kordofan.
People who spoke to the campaign group said forces aligned with Khartoum are systematically killing civilians believed to be opposed to the government.
The US President Barack Obama has already expressed his concern at reports of attacks in South Kordofan based on ethnicity.
Khartoum denies the allegations, and says it is fighting a legitimate war of self-defence against insurgents.
Many residents of the Nuba mountains fought with southern rebels during the two decade north-south war but now find themselves in the north.
South Sudan's President Salva Kiir said he would work with Sudan's President Omar al-Bashir to ensure their rights were respected.
Mr Bashir has recognised South Sudan but says 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.
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).
The ICC has 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.
The spokesman for the Sudanese Armed Forces, Colonel Khalid Sawarmi, told the BBC this was not true, and civilians hadn't been killed inside Kadugli.