The Sudanese army has launched repeated air strikes on southern army positions in Unity state, less than a month ahead of the south\'s independence, in a bid to seize the state\'s oil fields, a southern army spokesman charged on Friday. \"SAF aircraft bombed the area of Yau, in Unity state, many times on Thursday,\" Philip Aguer told AFP, referring to the north\'s Sudanese Armed Forces (SAF). \"This area is deep inside south Sudan and is a move by Khartoum to control the area and create a de facto border to control our oil fields,\" added the spokesman for the Sudan People\'s Liberation Army of the south. Aguer said the SPLA was on \"maximum alert\" and strengthening its defensive positions, fearing the start of an invasion to seize the oil fields. A Sudanese army spokesman was not immediately available for comment but the spokesman for a southern rebel group in Unity state, which has battled with the SPLA in recent months, confirmed the air strikes. \"The bombing was of SPLA positions, sending the SPLA running from South Kordofan,\" Bol Gatouth Kol said, while denying his group was involved in the fighting. Heavy clashes between SAF troops and northern members of the former southern rebel army first erupted in South Kordofan, the adjacent state north of the border, on Sunday. The heavily armed state retains strong links to the south, especially among the indigenous Nuba peoples who fought on the side of the southern rebels, even though their homeland, the Nuba Mountains, lies in the north. The United Nations earlier warned that the fighting had spread right across the volatile border state, raising the prospect of direct conflict between north and south Sudan just ahead of southern independence on July 9. The UN office for humanitarian affairs said in Geneva on Friday that up to 40,000 people are now estimated to have fled the fighting just in the state capital Kadugli. South Kordofan is north Sudan\'s only oil-producing state. It accounts for around 25 percent of Sudan\'s total output of around 480,000 barrels per day, meaning Khartoum will see a sharp fall in its vital oil revenues when the south secedes, unless an amicable revenue-sharing agreement is reached. That now seems unlikely.