KHARTOUM, (Reuters) – A United Nations staff member was shot dead in Abyei, the territory disputed between Sudan and South Sudan, U.N. officials said on Wednesday.

Abyei’s peacekeeping mission (UNISFA) said one of its national staff, a Ngok Dinka from South Sudan, was killed on Tuesday.

Sudan and South Sudan split apart last year under a 2005 peace deal that ended decades of civil war but failed to resolve a number of contested issues including the exact position of their shared border.

Control of Abyei, rich in grazing land and with symbolic significance for both sides, is one of the most hotly contested issues.

A UNISFA officer said its staff member was shot “during the violence that occurred while UNISFA troops were trying to control Ngok Dinka demonstrators who attempted to damage the Abyei town mosque”.

An official at the U.N. humanitarian agency confirmed the man had been shot and killed. It was not clear what the man’s role was or who had shot him.

Most of the permanent settlements in Abyei are occupied by Ngok Dinka, part of the South’s largest tribe. For several months a year, Abyei is also used by Arab Misseriya nomads, a well-armed group that provided proxy militias for Khartoum during the north-south war.

Some 2 million people died in Sudan’s civil war, which was fueled by disputes of religion, ethnicity and oil.