Armed Arab nomads killed at least one person in a series of clashes in Sudan's contested Abyei region, stoking tensions at the start of an independence referendum in south Sudan, officials said on Sunday.
A small group of Misseriya tribesmen, travelling on motorcycles, launched two attacks on police posts in the central oil-producing area on Friday and Saturday, said a U.N. source who could not confirm any deaths.
Senior Misseriya official Mohamed Omer al-Ansary said there had been a clash, leaving an unknown number dead -- but insisted his men had been attacked first by southern soldiers.
Analysts have said Abyei is one of the most likely places that could see a resurgence of violence more than five years after a peace deal ended decades of north-south civil war.
Both north and south Sudan claim Abyei, a central fertile territory on their shared border used by both the Dinka Ngok tribe, associated with the south, and Arab Misseriya nomads, associated with the north.
Its status was left undecided in a 2005 peace deal that ended the civil war and set up a referendum on whether south Sudan should secede -- a vote that started on Sunday.
Abyei was also promised its own referendum on whether it should join the north or the south on Jan. 9. But preparations for that vote were left in limbo after Dinka and Misseriya leaders failed to agree on who was eligible to vote and where Abyei's borders lay.
Northern and southern leaders have said they are now trying to negotiate a settlement but there has been no sign of compromise from either side.
Abyei administrator Deng Arop Kuol, a Dinka, told Reuters Misseriya tribesmen launched two attacks on civilians near the village of Miokol, 18 km (11 miles) north of Abyei town, killing at least one on Friday and an unknown number on Saturday.
"The civilians fell into an ambush by the Misseriya. We are still waiting for details," he said. Kuol said the Misseziya had heard false reports the Dinka were planning to hold their own referendum on Sunday and were trying to intimidate voters.
Misseriya official Mohamed Omer al-Ansary said Dinka soldiers had attacked nomads on Saturday afternoon to stop them taking their livestock to pasture and the Misseriya had fired back.
"There were six Misseriya injured and many (soldiers) killed ... We didn't start the war but we defend ourselves," he told Reuters. Abyei is guarded by a combined force of northern and southern soldiers but there has been little real integration.
A U.N. source, speaking on condition of anonymity, said officials had confirmed attacks on two police units in Todach and Shejei by a mobile group of around 10 nomads.