Under a 2005 north-south peace deal ending Africa's longest civil war, southerners will vote in a plebiscite on independence in seven months, but the semi-autonomous region has accused the north of arming militias to destabilise it ahead of the vote, which most analysts believe will result in secession.

"Yesterday we ambushed the forces of Galwak Gai ... and we killed eight of his men and we captured 13," south Sudan's army (SPLA) spokesman Kuol Diem Kuol said on Thursday. "They killed one from our side and two were wounded."

The clashes happened in the oil-rich Unity state. Gai is one of at least two other militia leaders coordinating with a former top SPLA officer George Athor who turned rogue after losing in April's elections, citing fraud. Athor's troops are in the neighboring Jonglei state where French oil giant Total holds a largely unexplored oil concession.

"We are now in hot pursuit of Galwak," Kuol said. "He will be captured or he will be killed," he said, adding Athor — who has yet to make good on a threat to attack a main town last month — was in hiding.

Kuol said Sudan's intelligence services were supporting the armed groups in the south, after the northern army refused to help them.

"According to the (prisoners) they were armed by the national intelligence in Khartoum," he said.

Sudan's northern army denied the report.

"The intelligence services work under the army and this is not true at all," an army spokesman said. "No organ can give any support to anyone rebelling anywhere in Sudan."

Sudan's north-south war claimed 2 million lives, mostly through hunger and disease, and destabilise much of east Africa. The international community has urged the north and south governments to begin talks urgently to agree on the disputed border and oil shares post-referendum.