Sudan: Barack Obama calls for ceasefire
US President Barack Obama has called for a ceasefire in Sudan, following an upsurge in fighting in the South Kordofan region.
He urged both the north and south to "live up to their responsibilities" to prevent a return to civil war.
Thousands of people have been displaced in recent days of violence, which comes only weeks before South Sudan becomes independent.
Meanwhile, the UN has accused Sudan of hampering aid efforts.
Roadblocks manned by militia are preventing aid reaching thousands of people in need, the UN's refugee agency said.
The agency said it had appealed to the Sudanese government to allow planes to land at the main airport in the affected area, in Kadugli.
Khartoum carried out what the UN described as an "intensive bombing campaign" near the border on Tuesday.
Northern forces are accused of targeting the area's pro-southern groups, as oil-rich South Sudan prepares for independence next month.
"There is no military solution; the leaders of Sudan and South Sudan must live up to the responsibilities," Mr Obama said in a recorded audio message.
"The government of Sudan must prevent a further escalation of this crisis by ceasing its military actions immediately, including aerial bombardments, forced displacements and campaigns of intimidation," he added.
Meanwhile, the head of the worldwide Anglican church, Archbishop of Canterbury Rowan Williams, said the unrest was a major threat to the stability of Sudan.
"The humanitarian challenge is already great, and the risk of another Darfur situation, with civilian populations at the mercy of government-supported terror, is a real one," the archbishop said, according to AFP news agency.
Southern Information Minister Barnaba Marial Benjamin welcomed Mr Obama's remarks and said a well-planned disarmament, demobilisation and reintegration programme was needed instead of the current use of force.
The bombing in South Kordofan follows a deal for both sides to withdraw from the nearby disputed town of Abyei.
Clashes over the past month in Abyei and South Kordofan have raised fears of renewed north-south conflict despite a 2005 peace deal which paved the way for the end of decades of war.
Some 40,000 people are estimated to have fled their homes in South Kordofan, on top of some 100,000 in Abyei, which was seized by northern forces last month.
Aid agency offices have been looted, churches have been ransacked and buildings destroyed.
Talks on issues about the upcoming split between Sudanese government officials and representatives of the south are continuing following Monday's deal for both sides to withdraw from Abyei.
President Omar al-Bashir and southern leader Salva Kiir agreed that Abyei, claimed by both sides, would be demilitarised, with Ethiopian troops ensuring security.
No time frame has been published.
The north-south war ended with a 2005 peace deal, under which the mainly Christian and animist south held a referendum in January on whether to secede from the largely Arabic-speaking, Muslim north.
Some 99% of voters opted for independence. President Bashir said he would accept the verdict of the south, where most of Sudan's oil fields lie.
Both Sudan and the South are reliant on their oil revenues, which account for 98% of South Sudan's budget. But the two countries cannot agree how to divide the oil wealth of the former united state. Some 75% of the oil lies in the South but all the pipelines run north. It is feared that disputes over oil could lead the two neighbours to return to war.
Although they were united for many years, the two Sudans were always very different. The great divide is visible even from space, as this Nasa satellite image shows. The northern states are a blanket of desert, broken only by the fertile Nile corridor. South Sudan is covered by green swathes of grassland, swamps and tropical forest.
Sudan's arid north is mainly home to Arabic-speaking Muslims. But in South Sudan there is no dominant culture. The Dinkas and the Nuers are the largest of more than 200 ethnic groups, each with its own languages and traditional beliefs, alongside Christianity and Islam.
The health inequalities in Sudan are illustrated by infant mortality rates. In South Sudan, one in 10 children die before their first birthday. Whereas in the more developed northern states, such as Gezira and White Nile, half of those children would be expected to survive.
The gulf in water resources between north and south is stark. In Khartoum, River Nile, and Gezira states, two-thirds of people have access to piped drinking water and pit latrines. In the south, boreholes and unprotected wells are the main drinking sources. More than 80% of southerners have no toilet facilities whatsoever.
Throughout the two Sudans, access to primary school education is strongly linked to household earnings. In the poorest parts of the south, less than 1% of children finish primary school. Whereas in the wealthier north, up to 50% of children complete primary level education.
Conflict and poverty are the main causes of food insecurity in both countries. In Sudan, many of the residents of war-affected Darfur and the border states of Blue Nile and South Kordofan, depend on food aid. The UN said about 2.8m people in South Sudan would require food aid in 2013. The northern states tend to be wealthier, more urbanised and less reliant on agriculture.