KAUDA, Sudan — At the crest of a steep green mountainside in central Sudan, Gasim Ibrahim huddled in the shadow of a huge boulder, listening in fear for the whining drone of planes 10,000 feet overhead.

A bomb from one of those planes in mid-June killed his sister, Faozi Ibrahim, who was six months pregnant, so Gasim and his extended family — more than a hundred of them — climbed here for safety.

"I will stay here as long as the war continues," he said.

Nearby, Dr. Tom Catena, an American from upstate New York, also talked about the planes, Russian-made Antonov cargo aircraft. The crude bombs that Sudanese crews roll out the backs of the planes as they pass over civilian areas blew off the arms or legs of many of the wounded in the clinic Catena runs.

He said the intent to target civilians was clear: "The Antonov is not a military weapon. There is no way to get any accuracy."

On Saturday, Sudan will become two nations under a 2005 peace agreement that divides the country into a northern, Arab-run, half with its capital in Khartoum and the new nation of South Sudan, run by Africans, with its capital in Juba.

When the peace agreement was signed, hopes were high that it would end decades of bloody civil war. But in the past month, the fighting in one of Sudan's most obscure conflict zones, the remote Nuba Mountains in the state of South Kordofan, has put such hopes in doubt.

The Sudanese government has prohibited foreigners from traveling to the Nuba, blocking road access and banning all flights. But a small group of journalists, including a correspondent for McClatchy, made it into the region this week. What they found backs accounts by aid workers and others of ethnic cleansing that Susan Rice, the U.S. envoy to the United Nations, said "could constitute war crimes or crimes against humanity."

The Nuba were among the non-Arab northern communities who joined with the southern rebel Sudan People's Liberation Army, and its political wing, the Sudan People's Liberation Movement, during the war. But when partition takes place Saturday, their land will lie within Sudan.

In May, the Sudanese government ordered former rebel fighters here to disarm or move into South Sudan. Clashes broke out June 5 and spread quickly, but the genesis of the fighting remains hazy.

In interviews, refugees and the wounded described what appears to have been a government plan to exterminate Nuba political opposition leaders through door-to-door executions. That soon deteriorated into an indiscriminate slaughter of whoever was "black."

Meanwhile, a heavy aerial bombardment by government Antonovs, MiG jets and helicopter gunships rocked rebel military strongholds and civilian villages.

Abdulaziz Tajir, a 36-year-old teacher, said he was near the Tangal village market when bombs began to fall one evening. Ten died on the spot, all civilians. Tajir suffered a baseball-sized wound in the chest.

Saleh Nahar, 32, was in his house when a bomb struck Kauda village and threw shrapnel through his leg. "Every time I hear a plane, I smell the bomb," he said.

Most graphic are descriptions of helicopter gunships hovering over fleeing civilians, strafing the ground. Juma Fadul, 25, fled from a church Friday as a helicopter opened fire in Al Hamra. He was shot in the head, but survived.

"Government troops yell, 'Allahu akbar'" — God is great — "and 'Let us chase the black people to the mountains' when they attack," he said. "All our houses had been set on fire."

Photographs collected by Nuba and distributed to reporters showed graphic scenes of civilians dismembered, disemboweled or lying in pools of blood.

Members of the Sudan People's Liberation Movement's northern branch who've fled to Kauda say the government had distributed lists of active members of the movement to be intercepted and killed. Nuba members of the Sudan People's Liberation Army responded with force, even pushing into Kadugli, South Kordofan's capital. That's when government forces began targeting civilians indiscriminately, slitting throats and opening fire on gatherings in the street, witnesses say.

Casualty counts aren't clear, but "hundreds" seems to be a cautious figure. Several people interviewed claimed that they'd witnessed more 100 deaths themselves.

What makes all this more dreadfully real for the Nuba is that it's not the first time they faced extermination.

In the early 1990s, as Nuba increasingly joined with the rebellion in the south, the Arab government in Khartoum declared a "jihad" against the Nuba in a campaign of killing and displacement that academic and Sudan expert Alex de Waal, who's now an adviser to the African Union mediation team for Sudan led by former South African President Thabo Mbeki, has described as "genocidal in intent." It's estimated that 200,000 Nuba died.

Official international investigations into recent incidents are unlikely to have much effect. The International Criminal Court in The Hague, Netherlands, already has charged Sudan's president, Omar al Bashir, and South Kordofan Gov. Ahmed Haroun with genocide for similar actions in Darfur.

An effort by the African Union to broker a cease-fire has stalled, and the Nuba say they foresee a long struggle ahead. Young new Nuba recruits can be seen training with wooden guns.

So far, the tens of thousands of Nuba Sudan People's Liberation Army troops have held their own against government firepower. A Sudan People's Liberation Army officer coming from the front lines, who asked not to be named in order to protect his family from possible government retribution, said the southern army had taken 61 government garrisons since fighting began. "We are on the offensive," the officer said.

His account couldn't be independently verified, though Sudan People's Liberation Army control over most of the Nuba Mountains appears for now to be protecting many villagers from all but aerial attacks.

How long the southern army can hold out, however, is unknown. The greatest long-term threat may be hunger as villages remain abandoned and fields unplanted.

The Sudanese government has barred humanitarian aid, and women and children huddled among the rocks were chewing leaves from a baobab tree for food.

"People cannot continue like this," said the local chief, Nimir Lalu Kodi.

(Boswell is a McClatchy special correspondent. His reporting from Sudan is supported in part by Humanity United, a California-based foundation that focuses on human rights issues).

MORE FROM MCCLATCHY

Reports find military buildup at Sudan's north-south border

Violence is rising as date for Sudan's partition draws near

WikiLeaks cables show worry about Saudi oil security

Follow McClatchy on Twitter.