(CNN) -- The heads of Sudan's north and south will meet Sunday at an African Union summit amid escalating violence and international condemnation.
Sudanese President Omar Hassan al-Bashir and Southern Sudan's President Salva Kiir Mayardit will sit down for talks in a session mediated by former South African President Thabo Mbeki in Addis Ababba, Ehtiopia, according to an African Union statement.
The meeting comes after the United Nations human rights office said it had received "extremely worrying" reports of civilian casualties and massive displacement of people in Southern Kordofan state.
The White House condemned mounting bloodshed in fighting between north and south and called on the United Nations to investigate possible war crimes.
The north's Sudanese Armed Forces have been engaged in fierce battles with the Sudan People's Liberation Army in the border state of Southern Kordofan.
"The United States condemns reported acts of violence in Southern Kordofan that target individuals based on their ethnicity and political affiliation," said a statement Friday from Jay Carney, the White House press secretary.
"Accounts of security services and military forces detaining, and summarily executing local authorities, political rivals, medical personnel, and others are reprehensible and could constitute war crimes or crimes against humanity," Carney said.
As many as 40,000 of the 60,000 residents of the city of Kadugli, are believed to have fled their homes, according to the United Nations. The global body said the town of Dilling and several villages around Kadugli were also reported to be deserted.
Rupert Colville, spokesperson for the U.N. Office of the High Commissioner for Human Rights, gave an example of the kind of violence taking place. He said his office received a report of a 25-year-old man who, along with his eight siblings, decided to return to their home Wednesday morning to retrieve food and other items.
"But they were confronted by police forces who shot and killed two of his brothers," Colville said. "The fate of the other six is unknown. Eyewitnesses confirmed the incident."
Fighting has also erupted in the disputed border region of Abyei.
The people of Southern Sudan voted in a long-awaited January referendum to secede from Sudan and form a new country. The balloting was considered free and al-Bashir was praised internationally for vows of cooperation with the south, which will gain formal independence July 9.
But ahead of that date, many are concerned that Sudan could again plunge into the kind of civil war that engulfed the African nation for decades.
The White House urged al-Bashir's government "to consider carefully the consequences of its current actions in Southern Kordofan."
"Although the United States has demonstrated a commitment to forging closer ties with Sudan, grave violations of international humanitarian law as have been reported to take place in Southern Kordofan will negatively impact this process and put Sudan on a path toward deeper international isolation," Carney said.
The African Union statement said the summit Sunday will focus on Abyei, including the withdrawal of armed forces from the area and the dispatching of an African led international peacekeeping mission to provide security. Also high on the agenda is the creation of conditions for the speedy return of thousands of displaced people.