Juba , South Sudan ( CNN ) -- Clashes erupted Monday on the volatile border between Sudan and South Sudan in the run-up to a planned meeting between the nations ' presidents as part of a troubled peace process . South Sudan army spokesman Philip Aguer accused Khartoum of launching aerial bombing raids on troops in Jaw , a border area where their armies have clashed previously . He said Sudanese ground forces attacked a southern base near the Heglig oil field . `` They repelled the attack , they pursued them to Heglig . The SPLA is controlling part of Heglig , '' he said , referring to South Sudan 's army . Sudan denied that its air force bombed Jaw , and said the border clashes near Heglig were `` minor '' . Part of Heglig is disputed , along with much of the 1,200-mile border . Sudan President Omar al-Bashir is scheduled to travel to the southern capital on April 3 to meet with his counterpart , South Sudan President Salva Kiir , and sign a deal on border demarcation . Under that agreement , the countries would establish a joint committee and technical team to mark the border . Another agreement would guarantee the rights of citizens from each country living on either side of the border . The accords are considered landmark achievements , reached after eight months of negotiations that began after South Sudan broke away from Sudan and declared independence July 9 . The split , which came after two decades of civil war , left a raft of unresolved issues , including how much the landlocked south should pay to transport its oil through a pipeline running across Sudan . South Sudan shut down oil production in late January after accusing its northern neighbor of stealing $ 815 million of its oil . Sudan said it confiscated the crude to make up for unpaid fees to use the pipeline and processing facilities in its territory . Pagan Amum , the south 's chief negotiator , said the border and citizenship agreements could open the way to a deal to resume oil production . Both countries are suffering economically since the shutdown began . Upon independence , South Sudan took three-quarters of the formerly united country 's oil reserves , and oil accounted for 98 % of the new state 's revenues . Monday 's border clashes cast doubt on whether either country would honor any agreements . During talks mediated by the African Union , the countries also signed a nonaggression pact on February 10 aimed at bringing peace to the border region . Aside from clashes between government forces , both countries accuse each other of supporting militia or rebel forces in their territories . On Thursday , U.S. State Department spokesperson Victoria Nuland urged South Sudan to cease military support for rebels fighting the Khartoum government in Blue Nile state and Southern Kordofan state , where Monday 's clashes occurred . Southern officials have repeatedly denied providing support to the rebels . Nuland also demanded that Khartoum `` end aerial bombardments of civilian areas . '' The war has already sent more than 100,000 refugees across the border into South Sudan , and the United States and United Nations have warned that hundreds more could follow , fleeing hunger and violence .
Presidents of South Sudan , Sudan to meet April 3 . Clearly defining contries ' border is main point of talks . Both countries blame each other for harboring rebel fighters .