|Fighting broke out over the disputed town of Abyei region in central Sudan two weeks ago [EPA]
The White House has urged Sudan to "immediately" end the escalation of "reprehensible" violence on southern army positions less than a month before the south becomes the world's youngest country.
"The United States condemns reported acts of violence in Southern Kordofan that target individuals based on their ethnicity and political affiliation," White House spokesman Jay Carney said on Friday, calling for a ceasefire.
The statement was in response to reports of further attacks by northern forces on the Sudan People's Liberation Army of the south.
"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 in a lengthy statement, which also called on the United Nations to investigate the reports.
The perpetrators should "immediately halt these actions and be held accountable for their crimes," he added.
On Friday, the south said the north had killed three civilians in a bombing at a border town in the Unity state region.
"There has been an air bombardment by northern forces in Unity state, in the morning yesterday and again in the afternoon. Three people were killed in the morning," Philip Aguer, spokesman for the south's Sudan Peoples' Liberation Army (SPLA), said.
"We are expecting not only air attacks but also ground forces. Our forces near the border are on maximum alert and are expecting an attack any time," he added.
The south is preparing to secede on July 9 but there are fears of fresh fighting between the two long-standing rivals, especially after the north seized the disputed Abyei region two weeks ago.
Aguer said northern forces were moving from Abyei towards Unity state. The northern army has not yet commented on the claims.
However, a UN spokeswoman told the AFP news agency that the northern army had not struck inside southern territory.
"The place that they bombed was an SPLA assembly area, right on the north-south border. This is one of the disputed territories," Hua Jiang for the UN mission in Sudan said.
As many as 40,000 people may have fled fighting in Sudan's South Kordofan state, the UN has said.
"Of the Kadugli population, estimated at 60,000, between 30,000 to 40,000 people are now believed to have fled the town," Elisabeth Byrs, spokeswoman of the UN Office for the Co-ordination of Humanitarian Affairs, told reporters in Geneva on Friday.
Amnesty International announced a "humanitarian emergency" in the border state's capital of Kadugli, on Friday.
"The Sudanese government must urgently put a stop to these indiscriminate attacks, and in particular must stop the bombing of areas populated by civilians and allow humanitarian agencies access to deliver assistance to the civilian population," said Erwin van der Borght, Africa programme director for Amnesty International.
Clashes broke out in Kadugli after a police station was attacked on Saturday.
Officials from the northern and southern ruling parties have traded blame for who started the fighting.
Southern Kordofan, which is in northern territory, has been seen as a potential flashpoint in the build-up to the south's scheduled secession because it is home to many fighters who fought against Khartoum during the last civil war.
It is also Sudan's only oil-producing state and accounts for around 25 per cent of Sudan's total output of around 480,000 barrels per day.
Khartoum is likely to see a sharp fall in its vital oil revenues when the south secedes unless an amicable revenue-sharing agreement is reached.
Another 100,000 people, most of them ethnically southern Dinka Ngok farmers, have fled to the south from the contested Abyei border region nearby since it was overrun by northern troops on May 21, according to UN estimates.