North Sudan’s army, which fought the south in a decades-long civil war, told Reuters the southern helicopter purchase would be seen as a "threat" and a violation of the 2005 peace deal that ended the conflict.
The statements will raise concerns among analysts who have warned that both north and south Sudan’s armies have been re-arming in the build-up to the politically charged referendum, due in January 2011, and that there is a risk of a return to conflict between the former civil war foes.
Southerners were promised a vote on whether to secede in the 2005 Comprehensive Peace Agreement that ended Africa’s longest civil war, a conflict which killed 2 million people, drove 4 million from their homes and destabilised the whole region.
Relations between both sides have remained troubled and southern leaders have accused Khartoum of trying to disrupt the vote to keep control of the south’s oil reserves.
"In the very near future, we want to buy transport helicopters so that we are able to supply our forces during the rainy season. The terrain is very difficult in south Sudan," said southern army (SPLA) spokesman Kuol Diem Kuol.
He dismissed as "a great lie" reports in Sudanese state media and other outlets that the south had already placed an order for 10 helicopters from a Russian supplier. Kuol added the south had the right to modernise its former guerrilla force.
SPLA chief of staff James Hoth told Reuters the southern army had set out plans for an air force as far back as 2007 and would carry them out if southerners chose independence.
"Yes we are planning, but not now … If the vote is for unity, we will share the national air force. If there is a vote for separation we will build up our own aircraft."
A spokesman for the northern Sudan Armed Forces (SAF) said the south had already placed the helicopter order.
"This is a clear violation of the peace deal and also it is a threat … This is a violation because the government of south Sudan is an internal government and has no right to have separate international relations. It is buying helicopters and the contract is in the name of the government of Sudan."
The rules of the north-south peace deal ban either side from replenishing arms or ammunition without the approval of a Joint Defence Board, but analysts say that is being flouted.
The northern spokesman added the SPLA had no mandate in the 2005 accord to arm itself with an air force, but it could do what it wanted if the south split away after the referendum.
Separately, the SPLA said Uganda’s brutal Lord’s Resistance Army rebels killed eight people in a night-time raid on a village near Yambio, capital of south Sudan’s remote Western Equatoria State, on Saturday.
"They assembled these people in one place and started chopping their heads with their machetes. It is part of Khartoum’s policy of terrorising people," said Kuol.
Khartoum regularly denies southern accusations that it is backing the LRA and other militias to destabilise the region.
(Additional reporting by Opheera McDoom; Editing by Giles Elgood).