Deng Alor, a senior leader of the Sudan People's Liberation Movement, reported clashes in Abyei on Sunday, confirming reports that calm had returned to the district on Sunday after fighting the day before in which several people had been killed and wounded.
"If the National Congress (Party of the north) want peace, cooperation and benefits with the south, the way is to cooperate with the SPLM, and to accept the implementation of the agreement on Abyei," Deng Alor said.
He added that militias organised by the NCP had carried out attacks in Abyei and in Bentiu, another key oil-producing district on the border.
Alor was speaking as voting began on Sunday in a referendum that will decide whether south Sudan opts to remain part of the north or choose independence, fulfilling a key provision of a 2005 peace agreement between the two sides.
A vote in Abyei to choose whether it wants unity with the north or south was also part of the 2005 accord and due to coincide with the independence referendum. But polling there was indefinitely postponed after neither side was able to agree on who should be eligible to participate.
Hamid al-Ansari, a leader of the Misseriya tribe, confirmed clashes Saturday between his men and rival Dinka Ngok tribesmen.
"The fighting happened yesterday when our cows tried to go to the river to drink and were fired on by the Dinka Ngok," Ansari told AFP by phone from Abyei.
"One of our people was injured and one of the Dinka Ngok was killed," he said, adding that the Dinka then returned with reinforcements and heavy fighting ensued.
"Five of our people were injured, while we got 24 of their weapons," Ansari added.
Abyei's southern-appointed chief administrator Deng Arop Kuol confirmed the region was quiet on Sunday, after fighting in which several people had died.
"The fighting was yesterday and the day before yesterday," he told AFP, without giving more details on the victims.
Much attention in the run-up to the southern referendum had focused on Abyei, given the district's potential to create a new flare-up between north and south Sudan.
Dozens were killed and more than 50,000 displaced from the oil-rich district during clashes in 2008 that threatened to derail the peace process.
A spokesman for the United Nations peacekeeping force in Sudan said UNMIS was investigating the latest violence.
"We are aware of this incident in Abyei, however we don't have details. A UNMIS patrol is going this morning to the location of the incident... to get more information," Kouider Zerrouk told AFP.
The nomadic Misseriya tribesman, who fought for the north during the 22-year civil war, migrate each year to the border region looking for pastures for their cattle.
They had threatened violence if they were not allowed to vote in the Abyei plebiscite, while the pro-southern Ngok Dinka population had indicated they would take unilateral action over the poll delay, raising fears of renewed clashes between the two heavily armed tribes.