(CNN) -- Negotiations between South Sudan and Sudan's presidents to come up with a border security and economic agreement will stretch into a third day Tuesday in Addis Ababa, Ethiopia.
Facing intense international pressure, Presidents Salva Kiir of South Sudan and Omar al-Bashir of Sudan, along with their negotiating teams, couldn't come up with a peace deal late Monday.
Sudanese Foreign Ministry spokesman El-Obeid Al-Marawih said negotiating a buffer zone or demilitarized zone and the disputed territory of Abyei will be at the top of Tuesday's agenda. He said that these are the toughest issues discussed thus far.
"There were some difficulties facing the meeting today," South Sudan spokesman Atif Kiir said in a late-night news briefing Monday. "Those difficulties and issues will be discussed on Tuesday. Any success of this presidential summit will depend on Khartoum."
South Sudan formally became a nation in July 2011, after people in the southern part of Sudan voted for independence in a referendum agreed to by leaders in the Sudanese capital of Khartoum.
Significant issues divide the nations. Chief among them are the demarcation of the border between them, the demilitarized zone issue, and the transportation and processing of oil from South Sudan, which got around 70% of the formerly united country's reserves when it became independent.