Article published on the 2008-05-20 Latest update 2008-05-21 08:38 TU
The UN had brokered a tentative ceasefire on 18 May between the two sides, which was shattered when the clashes erupted.
Secretary General Ban Ki-moon urged both parties "to immediately observe the terms of the ceasefire... including the disengagement of forces," his spokesperson said on Tuesday.
Aid workers said the violence started when the Sudan People's Liberation Army (SPLA) attacked Abyei, a town in the middle of the disputed area that has been under government control.
The entire population of the town and those living on the outskirts fled. The UN had evacuated its staff based there last week.
The army commander in Abyei said that SPLA forces had tried to seize the town. "It was a very aggressive attack on us. They had six tanks and many machine guns on vehicles. They used all their weapons to occupy Abyei," Brigadier General Muntasir Sabier said.
"But we defeated them. Our reserve came and chased them out of Abyei," he added.
The highest-ranking politician based in Abyei blamed the government military for sending in reinforcements overnight. "The objective of the SPLA now was first of all to defend ourselves and the area, to defend the people," said Edward Lino.
He added there had been reports that many were wounded.
"The roots of this current violence are intimately tied to the inability of the parties after almost three years, to implement the Abyei protocol," said the US embassy in a statement, referring to the plans drawn up to administer the border area.
The Comprehensive Peace Agreement, signed in 2005 after 1.5 million people were killed in Africa's longest-running civil war gave the south regional autonomy and participation in the national government. A referendum is planned for 2011 but could be postponed due to recent delays in conducting a census.