The south is preparing to secede on July 9 and fears of fresh fighting between the two long-standing rivals grew after the north seized the contested Abyei region on May 21.

The UN said fighting between northern forces and southern-aligned armed groups in the north-run oil state of Southern Kordofan had spread to the tip of the southern Unity state and that tens of thousands may have fled the clashes.

Philip Aguer, spokesman for the south's Sudan Peoples' Liberation Army, said the northern military was trying to occupy areas near the border -- whose exact position is yet to be decided -- in an attempt to control the country's oil fields.

A spokesman for the northern army was not immediately available to comment. In previous statements, it blamed southern or southern-aligned forces for provoking fighting in Abyei, Southern Kordofan and elsewhere.

South Sudan voted to secede in a January referendum promised by a 2005 peace deal that ended decades of brutal civil war between the north and south.

That vote went more smoothly and peacefully than many analysts and humanitarian groups predicted, but a lack of agreement between the two sides on questions such as how to share debt and oil revenues has complicated the split.