Air strikes on Sudan's Southern Kordofan border state may have killed as many as 64 people and caused tens of thousands to flee, the United Nations said on Wednesday. The northern military has been fighting southern-aligned armed groups in Southern Kordofan -- a northern, oil-producing state on the ill-defined north-south border -- since 5 June, raising tensions as the south prepares to secede on 9 July.

Humanitarian organisations fear a mounting death toll in the state, which is home to many fighters who sided with the south against Khartoum during the last civil war.

"There is a growing sense of panic among some of the displaced populations who find themselves trapped by the ongoing violence and the ethnic fault lines," the UN Office for the Coordination of Humanitarian Affairs (OCHA) said in a report.

"Reports of sectarian violence against civilians targeting members of the different ethnic groups as well as widespread looting of property are inhibiting returns to villages and towns of origin, even after the fighting has ceased," it added.

OCHA said about 60,000 people were estimated to have fled and more were believed to be hiding in the mountains.

"Local sources in the state indicate that 64 people have been killed in aerial bombardments since fighting broke out," it added.

A spokesman for the northern military was not immediately available to comment. The army, which says it is fighting a rebellion in the state, has blamed southern-aligned fighters for provoking clashes and denied its actions are harming civilians.

Officials with the south's dominant party have said the fighting started after the north's army tried to disarm fighters, many from the Nuba mountains region.

The United Nations said on Tuesday a campaign of aerial bombardment was causing "huge suffering" to civilians and endangering aid work in Southern Kordofan.

The south voted to secede in January, the culmination of a 2005 peace deal that ended decades of civil war between the two sides.