At least 13 people have been killed during clashes between south Sudan's army and armed northern nomads.

The fighting is the latest flare-up in the oil-producing border region, escalating tensions ahead of April elections, an army spokesman said on Friday.

"On our side two were killed in action and two were wounded," Kuol Diem Kuol, a south Sudan army spokesman, told the Reuters news agency.

He said the Misseriya nomads were "completely defeated" and 11 were killed.

The attack on the oil-producing Unity state, which produces Sudan's light Nile Blend, happened on Wednesday, Kuol said.

The Misseriya are loyal to the Khartoum government in the north but graze their cattle to the south of the border with the autonomous region.

The south says the north armed some members of the tribe as a proxy militia during the country's civil war.

'Political motives'

Kuol said there were political motives behind the attacks.

"Somebody somewhere is pushing them [the Misseriya]. They have been used for many years," Kuol said.

"Some Misseriya are grazing freely in the south ... but these are people who are insisting to come in with guns."

The south's semi-autonomous government says the nomads are welcome but must leave weapons in the north. Herders say this leaves them vulnerable to wild animals and cattle raiders.

An agreement signed earlier this year between the herders and southern officials said that the nomads may now bring in five small guns to accompany large herds and three guns if they are moving with smaller groups.

But Kuol says the nomads were breaking this agreement and carrying too many guns.

Southerners are set to vote in a January 2011 referendum on secession from Sudan's north, which they accuse of oppression after a civil war that has raged on and off for more than five decades.

Sudan's north and south have not agreed on key issues ahead of the 2011 referendum, including defining their common border, citizenship and the rights of communities whose livelihoods traverse the frontier.