Rights groups asked Sudan on Tuesday to end the indiscriminate bombing of civilians in the country's main oil-producing state, South Kordofan, and grant access for humanitarian aid.
Fighting between the army and rebels broke out in early June in the state bordering newly independent South Sudan.
After a week-long investigation in the Nuba Mountains, New York-based Human Rights Watch and London-based Amnesty International said Sudanese army airstrikes had killed at least 26 civilians, injured more than 45 and forced tens of thousands to flee their homes.
"These are no longer allegations; these are findings of the world's two biggest human rights organisations," Donatella Rovera, senior crisis response adviser at Amnesty International, told Reuters.
A leaked UN report said the Sudanese army had carried out killings, arbitrary arrests and detentions, disappearances, abductions, attacks on churches and aerial bombardments that, if proven, might constitute war crimes and crimes against humanity.
"The findings are that the Sudanese armed forces have been indiscriminately bombing the civilian populated areas, killing and injuring civilians," Rovera said.
The Sudanese government has dismissed the UN report as "unfounded" and "malicious" and has said it will form its own committee to assess the situation in South Kordofan.
Khartoum accuses local armed groups, many of whom fought alongside the south during decades of civil war, of launching a rebellion to try to take control of the territory.
The state holds most of Sudan's remaining known oil reserves, after southern Sudan seceded last month, taking its oilfields with it.
"We repeatedly saw Antonov aircraft circling above civilian areas and on several occasions, almost daily, we saw them dropping bombs in a manner which is not just inaccurate, but indiscriminate," Rovera said.
The groups accuse Sudanese President Omar Hassan Al-Bashir, who is wanted by the International Criminal Court (ICC) for crimes against civilians in the western region of Darfur, of breaking the two-week ceasefire that he unilaterally announced last Tuesday.
In a joint press release the rights groups said the displaced communities, forced out of their homes by the repeated bombing, live in harsh conditions in caves, on mountaintops, under trees and in the bush far from towns.
"Tens and tens of thousands have been displaced and forced out of their homes. The ones that we saw in different parts of the Nuba mountains were living in absolutely dire situations," Rovera said.
"No shelter for many of them, or very thin plastic sheeting for a very small percentage of them, hardly any food. People told us they were eating leaves and berries," she added.
The groups interviewed witnesses who said government soldiers and militia shot people in the streets and carried out both house-to-house searches and stops at checkpoints using lists of names of Sudan Peoples' Liberation Movement (SPLM) supporters in the state capital, Kadugli, and other areas.
The witnesses also described the destruction, looting and burning of churches and homes, including the bulldozing of homes of known SPLM members.