* Southern army says north backing militias in south
* Ambush killed 12, injured 10
* Unity state disputed in April elections
By Jeremy Clarke
JUBA, Sudan, Dec 2 (Reuters) - Sudan&${esc.hash}39;s south on Thursday accused northern-backed militias of killing 10 soldiers and two civilians in an ambush that comes as north-south tensions rise ahead of a Jan. 9 vote on southern independence.
Both sides accuse each other of violating the 2005 truce that ended Africa&${esc.hash}39;s longest civil war in Sudan and massing troops along the still disputed border.
"An SPLA truck was overpowered after falling into a big ambush 25 kilometres (15 miles) north of Bentiu," said southern army (SPLA) spokesman Philip Aguer.
"Twelve people were killed in the gunbattle; 10 SPLA soldiers and two civilians," he said, adding the attack happened on Wednesday. Bentiu is the capital of the oil-producing Unity state in south Sudan.
"They are definitely militia backed from Khartoum," he said. "It was a very coordinated attack."
The northern army (SAF) denied the report.
"This is completely untrue -- it&${esc.hash}39;s not possible that we have anyone in Unity state," said SAF spokesman al-Sawarmi Khaled.
The north-south U.N. peacekeeping mission (UNMIS) confirmed the attack but could not identify the aggressors.
"We are aware that an SPLA truck with soldiers and their families was ambushed by an unknown group yesterday ... in Unity state," said UNMIS spokesman Kouider Zerrouk.
He said 12 people died and 10 were wounded and said UNMIS would conduct a land and air patrol to the area on Friday.
Unity has seen much internal conflict after April elections there were heavily disputed between competing senior members of the southern ruling Sudan People&${esc.hash}39;s Liberation Movement (SPLM).
Tension between north and south Sudan is at an all-time high ahead of the Jan. 9 vote which is expected to create the world&${esc.hash}39;s newest nation state. The south has twice accused the north of bombing its territory in the past month. Khartoum says the south is helping rebels from the western Darfur region fight its army.
Sudan&${esc.hash}39;s north-south civil war was fought over differences over ethnicity, oil, religion and ideology. It claimed some 2 million lives and destabilised much of east Africa. (Additional reporting and writing by Opheera McDoom; Editing by Jon Hemming)
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