A senior Southern Sudanese official on Wednesday accused northern troops of "ethnic cleansing" near the internal border between the predominantly Muslim north and the mostly Christian and animist south.
Northern military aircraft have been bombing southern supporters in Southern Kordofan state north of the border that will divide the country in two when the south gets independence on July 9.
Riek Machar, vice president of the government of Southern Sudan, told The Washington Times in a phone interview that Southern Kordofan risks a fate similar to Darfur's if the international community fails to put pressure on the government in Khartoum.
Sudanese President Omar Bashir has been indicted by the International Criminal Court on charges of genocide in Darfur in western Sudan.
"I am concerned that what is happening in Southern Kordofan is ethnic cleansing," Mr. Machar said.
"People are disappearing, and intellectuals are being arrested," he added.
About 60,000 people have been forced to flee Southern Kordofan, and many more are hiding in the Nuba Mountains as the aerial bombardments continue, according to the U.N. Office for the Coordination of Humanitarian Assistance. It estimates that 64 people have been killed in the airstrikes.
Fatahelrahman Ali Mohamed, Sudan's ambassador to the United States, dismissed Mr. Machar's allegations.
He accused the Sudan People's Liberation Army, a southern group, of instigating the violence by attacking the region with heavy artillery.
"The Sudanese army is committed to doing its job without affecting civilians," he insisted.
Southern officials say the violence in Southern Kordofan could spread to other parts of the country, including the Blue Nile state.
The violence in Southern Kordofan, coupled with fighting in the disputed region of Abyei, which straddles the north-south border, has raised concerns about the region getting engulfed in war weeks ahead of Southern Sudan's independence.
President Obama late on Tuesday said the United States is "deeply concerned about the crisis that is unfolding in Sudan, including the fighting in Southern Kordofan and the assaults on innocent civilians."
"The leaders of Sudan and South Sudan must live up to their responsibilities," he added.
In a message recorded for Voice of America, Mr. Obama called on the Sudanese government in Khartoum to prevent a further escalation of the crisis by "ceasing its military actions immediately, including aerial bombardments, forced displacements and campaigns of intimidation."
Mr. Mohamed, the Sudanese ambassador, complained about Mr. Obama's decision to single out the north.
"What we see from the U.S., to put pressure on one side, that will not help," he said.
"The U.S. must also hold the other side [the south] accountable for violations in Southern Kordofan and Abyei," he added.
Southern leaders previously had alerted the international community to the violence in Southern Kordofan; however, these concerns were not taken seriously until Mr. Obama made his comments on Tuesday, Mr. Machar said.
"We want two feasible states to coexist side by side. This is not possible if there is war," he added.
According to Sudan Now, a group of anti-genocide and human rights organizations, an undeclared war already has begun between the north and the south.
"The war between North and South Sudan has resumed due to the offensive military operations launched by Khartoum," said John Prendergast, co-founder of the Enough Project.
Mr. Obama said both sides must end the violence, allow the free movement of aid workers and relief supplies, and fulfill their commitments under the Comprehensive Peace Agreement to resolve their differences peacefully.
The agreement in 2005 ended two decades of civil war that killed an estimated 2 million people.
Mr. Obama reminded Sudanese leaders that, if they fulfill these obligations, the U.S. will take the steps it has promised toward normal relations with Khartoum.
"However, those who flout their international obligations will face more pressure and isolation, and they will be held accountable for their actions," he added.
Mr. Prendergast said the policy of offering incentives to the Sudanese government has failed.
The U.N. Refugee Agency, meanwhile, called on local and central authorities to allow air and road access for humanitarian agencies working with displaced people in Kadugli, the capital of Southern Kordofan.
Melissa Fleming, the agency's chief spokeswoman, said armed militiamen have set up roadblocks and are blocking the agency's access to crucial supplies and displaced persons.
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