Long-standing tensions between northern and southern-aligned groups along the shared border have come to a head less than a month before the independence of the oil-producing south.

Southerners overwhelmingly voted to declare independence from the north in a January referendum promised in a 2005 peace deal that ended decades of civil war with the north, fought over religion, ethnicity, oil and ideology.

A return to full north-south conflict could have a devastating impact on the region by sending refugees back across borders and creating a failed state in the south at birth.

The south's army said its troops clashed with northern soldiers in Abyei on June 15 more than three weeks after the northern military seized the fertile, oil-producing area.

A spokesman for the northern army said he had no information about any clashes on June 15.

The northern army has also been fighting southern-aligned armed groups in Southern Kordofan -- a northern, oil-producing state that surrounds Abyei on the ill-defined north-south border -- since June 5.