* Military tension increasing as referendum nears
* U.N. considers sending 2,000 new troops to Sudan (Adds diplomats on planned U.N. peacekeeper increase)
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, it would be the second time this month the north 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 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 buildup to the southern vote. Sudan's economy depends on oil, located mostly in the south, and the Khartoum government does not want to lose an important source of revenue.
ADDITIONAL PEACEKEEPERS
In New York, U.N. diplomats said on Wednesday the United Nations was considering sending 2,000 new troops to south Sudan to boost the 10,000-strong U.N. Mission in Sudan force to a maximum of 12,000 troops. [ID:nN24243278]
The plan, which would require official approval by the 15-nation Security Council, is a response to a request from the president of south Sudan, Salva Kiir, who asked council members to approve a U.N.-monitored buffer zone along the semi-autonomous south's border with the north.
If accepted by the council and Khartoum, it would take three to four months for all 2,000 new troops to arrive in Sudan, one Western diplomat said on condition of anonymity.
Highlighting the escalation of tensions in Sudan, a senior northern figure stepped up the war of words by accusing the south of declaring 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 from the northern National Congress Party, told Reuters on Wednesday.
Mahdi said the Darfur rebel Justice and Equality Movement had moved its forces to the south to receive training.
Another rebel leader, Abdel Wahed Mohamed el-Nur of the Sudan Liberation Army, was expected to visit the southern capital, Juba, in the coming days, his spokesman said, and other rebel chiefs have visited or live there.
South Sudan's army was not immediately available for comment on Mahdi's remarks, but it denies aiding rebels from Darfur. (Additional reporting by Louis Charbonneau at the United Nations; Editing by Giles Elgood and Peter Cooney)
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