The northern army carried out cross-border air strikes that killed three civilians and wounded 17 in south Sudan's oil-producing Unity state just a week before independence, a southern official said on Sunday.

"The attack happened yesterday (Saturday) in Pariang, from seven to eight in the morning. Three people were killed and 17 were wounded. They were civilians," the state's information minister Gideon Gatpan told AFP.

He said the bombing was carried out by the Sudanese Armed Forces (SAF).

But the SAF spokesman dismissed the claims.

"This information is completely wrong. Pariang is a part of the south, and we don't have anyone to fight in south Sudan," Sawarmi Khaled Saad said.

Unity state lies across the border from South Kordofan, where fighting has raged since early last month between government forces and militia aligned to the former rebel movement that now rules the soon-to-be-independent south.

The violence in the ethnically divided northern state has already displaced more than 70,000 people, according to UN estimates, some of whom have fled to Unity state.

The Sudan People's Liberation Army of the south confirmed the bombings and accused Khartoum of trying to annex territory in the oil-rich south.

"In reality... the regime in Khartoum is planning to annex some of the territories of southern Sudan and claim them," the SPLA's spokesman Philip Aguer told AFP.

The south has already accused the northern army of bombing civilians south of Jau, the lakeside town just north of Pariang county, on a disputed stretch of the north-south border, a charge Khartoum has denied.

The conflict in Sudan's central border region has escalated tensions between north and south ahead of Saturday's "divorce," as have disagreements over post-separation arrangements, especially restructuring the vital oil sector.