Sudanese President Omar Al-Bashir has ordered northern troops in South Kordofan to "cleanse" the border state of pro-southern rebels but Washington said it remained confident southern independence would go ahead as planned next Saturday.
US special envoy for Sudan Princeton Lyman acknowledged there were still problems to be sorted out as the south prepares to break away but he added that he did not believe the fighting between northern troops and militia allied to the southern former rebels would derail partition.
"I ordered the Sudanese Armed Forces to continue their operations in South Kordofan until they cleanse the state of rebels," Bashir said in a speech broadcast on state television on Friday, just hours after his return from an official visit to China.
Heavy fighting has raged in the embattled northern border state since 5 June pitting northern troops against militia aligned to the Sudan People's Liberation Movement, the ruling party in the south, prompting more than 70,000 people to flee their homes, according to UN estimates.
The conflict has dramatically escalated tensions between north and south Sudan in the run-up to southern independence.
Bashir's speech dealt a blow to the prospects of peace in South Kordofan that were raised by the signing of a preliminary deal between the Khartoum government and the northern branch of the SPLM in Addis Ababa on Tuesday.
The violence in South Kordofan erupted after Abdelaziz Hilu, former deputy governor and number two in the northern branch of the SPLM, came close to winning the state's governorship election in May, before he withdrew alleging fraud.
Bashir called Hilu a "criminal" responsible for the indiscriminate killing of civilians, and said he must face justice. "Abdelaziz Al-Hilu is a criminal and he will continue as a criminal until he faces justice," the president said.
Ahmed Haroun, the Bashir loyalist who was elected, is wanted by the International Criminal Court for suspected war crimes in Khartoum's eight-year conflict with ethnic minority rebels in Sudan's western region of Darfur.
Church leaders and activists say the army's campaign in South Kordofan forms part of a government policy of ethnic cleansing, targeting the Nuba peoples who fought with the southern former rebels during their 1983-2005 war with Khartoum, claims the government strongly denies.
The International Rescue Committee said on Thursday that many of the displaced were ethnic Nuba, who were often hiding out in the Nuba Mountains with no access to medical assistance, food and clean water.
"Authorities in South Kordofan are barring international aid agencies from entering the region and supply lines have been cut," Susan Purdin, director of the IRC's south Sudan programmes, said in a statement.
"We’re extremely worried about the safety and well-being of people who live there. We’re hearing stories of horrible atrocities."
The US envoy to Sudan acknowledged that a ceasefire had yet to be shored up following Tuesday's preliminary accord between the two sides and called on Khartoum to provide urgent access to civilians fleeing the fighting.
"There is a political framework agreement, but work is still going on on the cessation of hostilities, and we continue to press very hard for more humanitarian access to the people who've been displaced," Lyman said.
The envoy added that he still expected a "celebratory event" next Saturday when the south becomes independent following a landslide vote for secession in a January referendum.
"There are still a lot of problems out there, but I think it's come a long way," Lyman said.
"I do think 9 July is going to come off in the way it should as bringing to an end decades of civil war between the north and the south."