Clashes spread along the border between Sudan and South Sudan on Monday as Khartoum deployed troops and warplanes in a bid to regain control of the oil town of Heglig.
A South Sudanese military official said the clashes are a "terrible escalation" of the border conflict that has been raging since South Sudan seized control of the Sudanese town on April 11.
It is unclear how many people have died in the fighting but Southern army spokesman Colonel Philip Aguer said that Sudan's air force killed five civilians in aerial attacks over Heglig.
Col Aguer said the airstrikes also seriously wounded nine people and hit oil wells.
He added that the town of Bentiu in South Sudan's Unity State was hit and that the conflict has spread to several southern states bordering Sudan, including Western Bahr el Ghazal.
"There has been continued bombardment by Sudan armed forces," Col Aguer said.
"Our forces are now on maximum alert."
Egyptian Foreign Minister Mohammed Kamel Amr met Sudanese officials in Khartoum over the weekend in a bid to end the escalating conflict.
Sudanese President Omer Al-Bashir told Mr Amr that his country is not prepared to resume negotiations with the South until it withdraws from Heglig.
Speaking before going into talks with South Sudan officials in Juba on Monday Mr Amr said: "We have come to explore ideas and ways to try to reach a peaceful resolution between the two nations - we don't have a specific proposal yet."
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