Four Zambian members of the U.N. peacekeeping mission were shot and wounded while on patrol in Abyei by unknown assailants, the U.N. Mission in Sudan said.
"UNMIS strongly condemns this unprovoked attack and has commenced an investigation into the incident," the agency said in a statement.
The four were attacked about 15 miles north of the troubled region of Abyei just days after the northern and southern Sudanese governments recommitted to a peace deal over the area.
On Tuesday, an integrated force took over for security responsibility of Abyei. Leaders from both Sudanese governments early this year backed the Kadugli and Abyei agreements that called for the withdrawal of illegal forces in the area in favor in an integrated police unit.
In January, voters backed a referendum for an independent South Sudan. The referendum was part of a 2005 comprehensive peace agreement that ended a civil war in Sudan.
Abyei was left out of the process, however, because of voter eligibility issues. Sudanese President Omar al-Bashir reneged on earlier promises by saying he wouldn't recognize South Sudan if Abyei broke away from the north.