“The market was burning,” said Salah Kaka, a mother of four who trekked for days with thousands of others to a mushrooming refugee camp after her husband disappeared during an air raid. “I dug ditches in the ground and hid the children.”

Tens of thousands of rebel fighters have refused the government's threat to disarm, digging into the craggy hillsides. They are demanding political reform and autonomy, a familiar refrain in Sudan's marginalized hinterlands that has set off insurgencies in Darfur in the west, as well as eastern and southern Sudan.

“This is going to spread like wildfire,” said an American official who was not authorized to speak publicly. Without mediation, “you're going to have massive destruction and death in central Sudan, and no one seems able to do anything about it.”

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The Sudanese Army has sealed off the area and threatened to shoot down United Nations helicopters. Sudan’s forces detained four United Nations peacekeepers and subjected them to “a mock firing squad,” the organization said Monday, calling the intimidation part of a strategy to make it nearly impossible for aid agencies and monitors to work in the region.

It seems that the Sudanese government, facing upheaval on several fronts, especially with the southern third of the country preparing to declare independence next month, is determined to suppress the rebels and prevent them from encouraging other restive areas to rise up.

Even after the southerners secede, countless fault lines remain in northern Sudan. Non-Arab people in the Nuba Mountains, Darfur, Blue Nile State, Kasala — and all the way down the Nile to Egypt — have long been chafing against an increasingly isolated government dominated by a small group of Arabs and led by President Omar Hassan al-Bashir, a war crimes suspect indicted by the International Criminal Court.

“Bashir is facing enormous pressure,” said EJ Hogendoorn, a program director at the International Crisis Group. “There are a number of areas that could rebel again,” he said, and the offensive in the Nuba Mountains “may actually exacerbate resentment and inadvertently unite armed opposition movements.”

United Nations officials in Southern Kordofan, the state that includes the Nuba Mountains, estimate that dozens have been killed in aerial bombings in the past two weeks and maybe dozens more in extrajudicial killings. Nuban officials put the civilian death toll in the hundreds.

Sudanese soldiers are planting land mines in several towns, United Nations officials said, and possibly digging mass graves. Many people in the mountains are Christian, and church officials say Christians have been attacked and churches burned.

“So many people have been made to leave their homes,” said Ali Shamilla, liaison officer for the Nuba Relief, Rehabilitation and Development Organization. “Many are living in caves.”

Witnesses said government soldiers were shooting “the black people,” a reference to Nubans, who are often darker skinned than the Arab-dominated military. Human rights groups worry that this could begin a new round of ethnic cleansing, given the wholesale destruction of communities that has been part of how war is fought in Sudan.

Hundreds of thousands died in Darfur after the government razed villages and armed militias to throttle rebels there, leading to genocide charges against Mr. Bashir. Millions died in the decades of civil war between north and south, under many of the same tactics.

The same thing happened in Nuba. In the mid-1980s, southern rebels opened bases in the Nuba Mountains. Residents who had long felt discriminated against by the Arab rulers of Sudan joined the southerners in droves.

The rulers responded by arming Arab militias — just as it would in Darfur — and setting them loose on impoverished villagers. Tens of thousands of civilians were killed and villagers were incarcerated in “peace camps,” forced to convert to Islam. Entire villages were wiped out.

“Nuba were often just shot on sight by Khartoum forces, no questions asked,” said Roger P Winter, a former State Department official, who testified Thursday during a Congressional hearing on Sudan's future. “Today, again, Nuba are positioned for liquidation by Khartoum forces.”

This may sound hyperbolic. But as Julie Flint, an author who first visited the Nuba area in 1992, argued, some of the same men responsible for earlier atrocities in Nuba are in charge once again, including Ahmed Haroun, the Southern Kordofan governor, indicted by the International Criminal Court for crimes against humanity connected to Darfur.

“A new war in Nuba threatens to be a replay of Darfur,” Ms. Flint said.

The Sudanese government does not deny bombing Nuban villages, arguing that the Nuba militia were supposed to disarm but did not. One Sudanese official said the war could go on “for some years.” Nuban militia leaders have vowed to fight until there is “regime change” in Khartoum or autonomy for Nuba.

Under the accords that set in motion the south's secession, Nubans were supposed to hold “popular consultations” to determine their future, but that has not happened. Now that the south is on the verge of realizing its hard-fought goal — independence — many Nubans feel their demands have been deferred.

In the north, oil had helped buy friends and woo enemies, but huge economic uncertainties loom. The south has most of the oil, and in any deal before the south splits off, the north will almost certainly get less than it used to.

Already, riots have broken out in central Sudan's Arab heartland, as Mr. Bashir has warned of austerity measures. Many analysts say the recent military activity along the north-south border, including the north's seizure of the disputed Abyei area and its push in the Nuba Mountains, is part of a hard-knuckled negotiation to secure more oil revenue.

Southern Sudan's leaders are reluctant to go to war over Nuba, but the southern-allied militiamen in Nuba are part of the overall southern military command, so the south could be dragged into the conflict.

During a recent meeting, the top Nuban militia commander, Abdel Aziz al-Hilu, said that before any cease-fire he would have to inform “Chairman Salva,” meaning southern Sudan's president, Salva Kiir. Mr. Abdel Aziz also said that if things don't change, “fires will just break out everywhere, here, in Blue Nile, in Darfur,” according to someone at the meeting.

“We, the people of Sudan, are ready to remove them,” vowed Mr. Abdel Aziz, the person said. “We have guns.”