* Southern army says north looking for "pretext" for war
* Northern minister threatens to reject key referendum
* Both sides dismiss each other's accusations
(Recasts with northern and southern accusations)
By Andrew Heavens
KHARTOUM, Sept 27 (Reuters) - Northern and southern Sudanese officials traded accusations on Monday of deploying troops along their joint border as tensions mounted in the build-up to a referendum on southern independence.
South Sudan's army accused the north of building up around 70,000 troops in contested areas and plotting an invasion of the oil-producing south.
Earlier a northern government minister told journalists the south's army had strayed into disputed territory, saying his dominant National Congress Party (NCP) might reject the result of the referendum unless the soldiers moved.
Both sides dismissed each other's accusations.
Sudan's Muslim north and the south, where most follow traditional and Christian beliefs, ended decades of civil war with a 2005 peace deal that allowed them to keep their own armies.
Analysts say trust between the two former foes has hit an all-time low in the countdown to the referendum, due on Jan. 9, that will allow southerners to choose whether to secede or stay in Sudan, a vote set up as the climax of the peace process.
The latest accusations come days after northern and southern leaders met U.S. President Barack Obama and other world leaders during the U.N. General Assembly and promised to hold the potentially explosive vote peacefully and on time.
Haj Majid Suwar, Sudan's youth and sports minister, accused the south's army of straying out of areas assigned to them in the peace accord, and said southern authorities were cracking down on supporters of Sudanese unity.
Asked what the NCP would do if the southerners did not allow open campaigning and move their troops, he said: "We will talk to ... the USA and the U.N. and the AU (African Union) and say that the other side, they didn't fulfil the CPA, so we may not recognise the results."
Southern army spokesman Kuol Deim Kuol dismissed the accusations as "baseless".
"Why do they turn up with this statement at this time? ... This is someone who is looking for a pretext of starting the fight. This makes us conclude that the NCP and their SAF (the northern army) have preparations to reoccupy the south."
"According to our intelligence estimates, there are seven (northern) divisions dotted along the border," he added. "We have been expecting that by October they will do something." Each division, he said, amounted to around 10,000 men.
A northern army spokesman denied his forces had any aggressive intentions against the south. "This is absolutely wrong and there is no evidence," he said.
The two sides have clashed since the signing of the peace accord, most recently two years ago in Abyei, a disputed central region with oil reserves and rich pasture.
Analysts have warned there is a risk conflict could reignite if the referendum -- highly prized by southerners who are expected to vote for separation -- is disrupted or delayed. (Additional reporting by Khaled Abdel Aziz)
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