United Nation peacekeepers warned Tuesday violence and looting in South Sudan's conflict-wracked Pibor region of Jonglei state were growing worse amid rebel and army clashes.

The United Nations Mission in South Sudan (UNMISS) is "deeply concerned about the deteriorating security situation in and around Pibor town", it said in a statement, two days after gunmen were reported to have looted a hospital there.

The UN "strongly condemns the violence, looting and displacement affecting civilians and humanitarian organisations," it added, noting it was "particularly alarmed" at reports it involved "ill-disciplined members of security forces."

South Sudan's rebel-turned-official army has been fighting to crush a rebellion in the Pibor region of the eastern state of Jonglei.

The rebels, led by David Yau Yau, a former theology scholar fighting since April 2011, call themselves the South Sudan Democratic Movement/Army (SSDM/A).

The group issued a statement last Monday boasting that Pibor was "on the verge of falling into our hands", although UN peacekeepers remain in the town.

Humanitarian sources told AFP on Sunday that uniformed men ransacked and looted a hospital and premises of French aid agency Doctors Without Borders (MSF).

The men also "flattened" premises of the Italian aid agency INTERSOS and raided UN stores in Pibor town late on Saturday and Sunday.

Yasmin Haque, UN's top humanitarian official in the country, said that aid agency "compounds were completely looted" with fixtures such as solar panels stolen, even though they were nearby to the army's barracks.

The volatile eastern state of Jonglei has been the scene of widespread ethnic conflict since South Sudan became independent in July 2011, with bloody battles between rival tribes, including the Dinka, Lou Nuer and Murle people.

The region is still reeling from a 1983-2005 civil war that left communities awash with guns and riven by ethnic hatred, with traditional cattle raiding between rival tribes escalating into a wave of brutal killings.