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JUBA, Sudan (Reuters) - Clashes between South Sudan's army and northern nomads have killed at least 18 people, an army official said on Friday, highlighting the need to demarcate the border along which much of Sudan's oil lies.

Western powers have emphasized the need to agree on the north-south Sudan border ahead of elections in April and a key southern referendum on independence, which most analysts expect to result in secession in 2011.

"It was a surprise attack. They managed to kill seven -- six soldiers and one civilian," South Sudan's army spokesman Kuol Diem Kuol said of the latest attack on Wednesday.

He added that eight bodies from the attacking Misseriya, northern Sudanese cattle keepers who often graze their herds in the south, were also found in the oil-producing Unity state. An earlier attack killed three from both sides.

After a 2005 peace deal ending more than two decades of north-south civil war, Sudan's former foes formed an uneasy coalition government but relations are tense over delays in the democratic transformation envisaged by the deal.

Talks to agree the border and other post-referendum arrangements are stalled, worrying many Sudanese as well as the international community who fear conflict could reignite if the frontier is not agreed before the 2011 vote.

"The (Misseriya) want to graze their cows and the locals are resisting. So it's a border conflict in short," William Keah, a local Unity state official said.

South Sudan recognizes the rights of the Misseriya to graze in the south but says they must leave their weapons behind, so they do not clash with southern tribes whom the government is trying to disarm.