The Sudanese armed forces burned and shot civilians to death in a "scorched earth" campaign against a rebel chief's home district in Blue Nile state, Amnesty International said on Tuesday.

Satellite imagery confirms that the attacks in Blue Nile's Ingessana Hills, the birthplace of rebel chairman Malik Agar, occurred in the first half of last year, the London-based watchdog said in a 74-page report.

Sudan's army called the charges a fabrication.

The attacks were part of what appeared to be "a concerted attempt" to clear civilians from areas held by Agar's Sudan People's Liberation Movement-North (SPLM-N), and to punish residents perceived to be supporting the rebels, the rights group said.

The Ingessana area, southwest of the state capital Ed Damazin, was particularly hard-hit, Amnesty said, after visiting rebel-held areas and interviewing refugees.

About 150,000 people have fled to South Sudan or Ethiopia since fighting began in September 2011.

"The army used scorched-earth tactics, destroying at least eight villages in the (Ingessana) area and probably many more," Amnesty said.

"Sudanese forces would bomb and shell villages before invading and burning them down" after using indiscriminate firepower, it said.

"Civilians fled when the attacks began, but some of those who were unable to flee because of disability or age were burned alive in their homes or shot by soldiers."

Amnesty's Sudan researcher, Jean-Baptiste Gallopin, said in a statement that deliberately attacking civilians is a war crime.

The report quoted Mugos Masim, 36, a resident of Ingessana's Qabanit village, as saying the army used "huge Dushka machine guns, shooting at each house."

Faki Moul, Qabanit's deputy sheikh, added: "From the mountain I saw the whole village burning."

A similar attack occurred against Khor Jidad village, the report said.

It quoted a resident, Shaybou Osman, 30, as saying they found the bodies of an old man and two children, one aged about seven, who had been shot in the back.

"The bullet had come out of his stomach and his intestines were hanging out," Osman said.

Sudanese army spokesman Sawarmi Khaled Saad told AFP that Amnesty's allegations were illogical.

"There are many NGOs working in Blue Nile and they did not make such a complaint," he said. "This is a fabrication."

Rebels control less than 10 percent of Blue Nile, he added.

Sudan has severely restricted access to Blue Nile for foreign aid workers, journalists and diplomats.

Similar restrictions apply in South Kordofan state where the SPLM-N is also fighting.

Amnesty said that refugees who reach camps in South Sudan face the threat of "coercive recruitment" by the SPLM-N.

The SPLM-N was allied with the SPLM, the now ruling party of South Sudan, during its 1983-2005 war with Khartoum that culminated in independence in 2011.