David Gressly, who heads the southern section of the United Nations Mission in Sudan (UNMIS), said: ”The landmines have been set in a pattern capable of taking out convoys.”
Fighters loyal to renegade southern general George Athor are accused of laying the explosives in northern Jonglei state, Gressly told reporters in the southern regional capital of Juba.
UN deminers were travelling to investigate and support clearance, he added.
South Sudan has seen an upsurge in bloody clashes between rebel groups and the army since a largely peaceful January referendum on independence in which southerners voted almost unanimously to form their own nation.
The violence has left hundreds of people dead, many of them civilians.
Gressly said that peacekeepers had now gained access to almost all areas blocked during the fighting in recent weeks.
“We have gained access to many areas where access had been curtailed,” he said, adding that there were at least four rebel groups operating in pockets in the neighbouring states of Jonglei, Upper Nile and Unity.
However, he admitted a few areas remained difficult to get into – some due to the risk of landmines and others due to restrictions by southern army field commanders.
Earlier this month, southern officials released documents they said detailed northern arms shipments to southern militias.
The north has repeatedly rejected such accusations, insisting the documents are forgeries.