Sudan seen massing troops in oil state, U.N. staff blocked
South Sudan to secede on Saturday but oil state conflict threatens stability after split
North Sudan wants peacekeepers out after south split
A U.N. exit would be devastating for civilians in disputed areas, a senior aid official said
Sudan's division echoes in war-ravaged Darfur
Many fear south Sudan's split will worsen fighting in the war-torn Darfur region