* South accuses north of air raid
* Military tension increasing as referendum nears
* North says south supporting Darfur rebels
By Opheera McDoom
KHARTOUM, Nov 24 (Reuters) - Sudan's south accused the northern army of carrying out an air strike on an army base in southern Sudan on Wednesday in an attempt to derail a Jan. 9 referendum on southern independence.
As the plebiscite approaches, leaders of north and south Sudan have accused each other of building up troops in the border region. While the south is seen likely to vote for secession, the north would like to keep the country whole.
If confirmed, this would be the second time this month the north has conducted aerial raids in the south. On the first occasion, the southern army said northern forces accidentally dropped a bomb on its territory while fighting rebels from Darfur near the north-south border.
"Today SAF (northern army) helicopter gunships attacked our position, injuring four soldiers and two civilians," southern army spokesman Philip Aguer said.
"The SAF is trying to drag Sudan back into war again and to disrupt or prevent the referendum," he said, but the southern army would not respond militarily.
The SAF denied it had attacked the south on Wednesday.
"This is absolutely not true. We have not attacked anywhere near the border," said SAF spokesman al-Sawarmi Khaled.
A U.N. spokesman had no immediate information on the report.
Kuol Athuai, the commissioner for the area of the attack, Aweil North, told Reuters by telephone: "This was the northern army -- they attacked a village and an army base."
Relations between north and south Sudan have been tense in the build-up to the southern vote. Sudan's economy depends on oil, mostly located in the south, and the Khartoum government does not want to lose an important source of revenue.
In an escalation of the pre-referendum war of words, a senior northern figure said the south had declared war by supporting rebels from Darfur.
"If you are accommodating these forces in the south, you are supplying these forces with weapons, logistics, petrol and cars ... we think that this is a declaration of war against the north of the country," Mandour al-Mahdi, a senior official from the northern National Congress Party, told Reuters on Wednesday.
Mahdi said the Darfur rebel Justice and Equality Movement (JEM) had moved its forces to the south to receive training.
Another rebel leader, Abdel Wahed Mohamed el-Nur of the Sudan Liberation Army (SLA), was expected to visit the southern capital Juba in the coming days, his spokesman said, and other rebel chiefs have visited or reside there.
South Sudan's army was not immediately available for comment on Mahdi's remarks, but it denies aiding rebels from Darfur. (Editing by Giles Elgood)
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