* South Sudan army wants U.N. to monitor troop movements
* Says tens of thousands of northern troops moved south
* Says north denying U.N. access to investigate
By Opheera McDoom
KHARTOUM, Oct 19 (Reuters) - South Sudan's army accused the United Nations on Tuesday of failing to report a massive build-up of northern troops around sensitive border areas, warning war could erupt.
Senior southern army officer Mat Paul said the separate north Sudan army was preventing the U.N. peacekeeping mission (UNMIS) monitoring a ceasefire and U.N. officials were not owning up to their lack of access.
Most analysts believe southerners will vote to secede from the north in an emotional referendum on independence due in less than three months, the culmination of a 2005 north-south peace deal ending Africa's longest civil war.
"This year, the build-up of SAF (northern army) started in June in South Kordofan and other areas and we've been raising this several times with the U.N." said Paul, who is the SPLA's representative in the joint north-south ceasefire monitoring commission (CJMC) chaired by the United Nations.
"They...just keep quiet so there is no monitoring," he said.
Sudan's north-south civil war pitted Khartoum's Islamist government against rebels who mostly followed Christianity and traditional beliefs. The conflict was complicated by oil, ethnicity and ideology. It claimed an estimated 2 million lives.
The more than 10,000-strong UNMIS force is mandated to monitor the 2005 truce but has in the past had problems accessing sensitive areas.
SENSITIVE BORDER AREAS
The northern army has denied moving its forces south but was not available to comment on denying U.N. access.
Paul told Reuters the north had moved tens of thousands of troops south into areas including oil-producing Heglig and Unity, and other sensitive border areas including South Kordofan, Abyei, Raja -- a site of previous clashes.
He said the atmosphere was very tense with the soldiers accompanied by tanks and helicopters stationed nearby. "The troops look to be on the verge of war," he added.
The senior SPLA officer said they had raised SAF's troop build up in three consecutive CJMC meetings since August but the U.N. mission had told them they could not investigate because the northern army would not allow them.
The UNMIS spokesman was unable to comment. UNMIS chief Haile Menkerios said on Monday that there was "overall tension" but the peacekeepers had not investigated the troops amassing because they had heard the complaints only in the media.
"I really regret this is what the UN is saying -- it is not correct," said the SPLA's Paul. "They need to double up their efforts in order to stop the parties going back to square one."
A U.N. spokesman in New York, Farhan Haq, said UNMIS was redeploying troops to hotspots including Abyei but declined to comment on whether UNMIS was able to investigate troop movements in violation of the ceasefire.
UNMIS came under close scrutiny following allegations it did not do enough to protect civilians during north-south clashes in the disputed Abyei border region in 2008.
(Additional reporting by Louis Charbonneau in New York; Editing by Ralph Boulton)
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