Radia Achouri, spokeswoman of top UN envoy in Sudan Jan Pronk, told a press conference that a camp for displaced people in the southern Juba town was attacked by an armed group thought to belong to the Lord Resistance Army (LRA), Ugandan rebel militia active in southern Sudan.
She said eight displaced people including three women and two children were reportedly dead.
The UN would respond to a demand by the Sudanese government and the Sudan People's Liberation Movement (SPLM) to send experts to investigate the incident, she added.
The UN official slammed the LRA as "a real hampering factor against stability in southern Sudan", adding that the international peacekeeping forces in Sudan had the right to defend themselves against any such attacks.
The Sudanese government and the SPLM reached a comprehensive peace deal in January, ending a 21-year-long civil war.
In addition, Achouri said banditry still rampaged in Sudan's conflict-ridden western Darfur region.
She said criminal gangs had frequently targeted vehicles transporting humanitarian aid and killed the drivers.
Violence flared up in Sudan's arid Darfur region in February 2003 as rebels took up arms against the Sudanese government, accusing it of neglect.