Obama calls for Sudan ceasefire

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," 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.

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 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.