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.