By Amber Henshaw
BBC News, Khartoum
The Sudanese government has missed a key deadline under the Comprehensive Peace Agreement (CPA) to withdraw its troops from the south of the country.
The CPA included protocols on sharing power and wealth
Only two-thirds of the northern Sudanese Armed Forces (SAF) have been redeployed, officials have said.
Many fear their continued presence will lead to renewed tensions as the South takes over its own security on Tuesday.
The peace agreement signed in January 2005 ended a 21-year-old civil war in which 1.5 million people died.
The conflict pitted the mainly Muslim North against the Animist and Christian South, led by the rebel Sudan People's Liberation Army (SPLA).
The redeployment of soldiers by 9 July was one of the deals made under the CPA.
Both sides have been slow to move their troops.
A UN statement signed by the UN, SAF and SPLA on Monday said that only 66.5% - just over 30,000 soldiers from the North - had been redeployed.
The status of another 9,000 troops is contested by the South.
The head of the United Nations Mission in southern Sudan (UNMIS), Peter Schumann, told Reuters news agency that most of the remaining northern forces were in oil production areas.
Under the peace agreement, the protection of Sudan's oil fields should fall to special, integrated units made up of both armies.
But Mr Schumann said it was unclear how long it would take for these special units to take up the role.
The presence of northern troops in the South is likely to cause tensions once the southern government takes over control of security in the region on Tuesday.
Another key issue still to be resolved is that of the militia backed by the northern government.
Under the CPA, they were supposed to be absorbed into the Sudanese Armed Forces, the southern army or demobilised.
The North claims they have been integrated, but this is contested by the SPLA.
There are concerns that if the militia problem is not resolved, it could lead to fighting.
Last November, there were hundreds of deaths in clashes between the two in the southern town of Malakal.