WASHINGTON (AFP) – President Barack Obama is calling on Sudan's warring factions to end the bloody violence threatening a peace agreement as the south gears up for independence due in three weeks.

"There is no military solution," Obama said in an audio message recorded late Tuesday for the publicly-funded VOA broadcasting network.

"The leaders of Sudan and South Sudan must live up to their responsibilities. 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."

His appeal for a ceasefire, taped after returning to the US mainland from a visit to Puerto Rico, came amid a surge of violence that has seen more than 1,500 people killed in south Sudan since a largely peaceful independence referendum in January, according to UN and official figures.

Heavy fighting has swept across South Kordofan state on the south Sudan border since June 5 pitting the northern Sudanese Armed Forces and allied militiamen against fighters aligned with southern former rebel group the Sudan People's Liberation Army.

Other clashes in south Sudan have killed almost 100 people in separate cattle raids and rebel attacks in the past week, officials said Wednesday.

Obama called on both sides to agree to end violence, allow aid workers and relief supplies to move freely, fulfill their commitments under the 2005 Comprehensive Peace Agreement that ended a 21-year civil war and "resolve their differences peacefully."

"Today, I want to speak directly to Sudanese leaders: you must know that if you fulfill your obligations and choose peace, the United States will take the steps we have pledged toward normal relations," he continued.

"However, those who flout their international obligations will face more pressure and isolation, and they will be held accountable for their actions."

The Khartoum government in the north is keen to score normalization, including being removed from a US list of state sponsors of terror.

"The Sudanese people have come too far, and sacrificed too much, to see their dreams of a better future slip from their grasp," Obama said.

"Now is the time for Sudanese leaders to show the courage and vision that true leadership demands. Now is the time for Sudanese leaders, north and south, to choose peace."

The south is due to proclaim full independence on July 9, under a peace deal after decades of conflict with the north, but the fighting threatens to overshadow the historic event, particularly if the southern army is drawn in.