JUBA, South Sudan: The United Nations Friday warned of rising instability in South Sudan's Jonglei state which has been the scene of recent large-scale massacres, alleged army abuses and an ongoing rebellion against the fledgling government.

"These reoccurring spates of violence are extremely serious and can jeopardize everything that has been achieved so far for peace and reconciliation in Jonglei," the head of the UN Mission in South Sudan (UNMISS) Hilde Johnson said in a statement adding that she was "very worried" about the cycle of violence.

The statement followed a visit to Walgak, a region in northern Jonglei, where a recent attack by what the government called a group "armed to the teeth" led by David Yau Yau left over 100 people dead.

South Sudan split from the north in July 2011 after decades of civil war. The legacy of guns within communities that Khartoum often pitted against each other has turned centuries-old cattle raiders increasingly deadly.

At just six months old, South Sudan witnessed its most violent incident since the war ended in 2005. Up to 8,000 armed youths from one tribe in Jonglei marched on Pibor county, home to the minority Murle people, killing over 600, the UN says.

Local death tolls were in the thousands and medical emergency charity

Medecins sans Frontieres (MSF) voiced concern about the increasing brutality in attacks and targeting of women and children.

An army-led state wide disarmament campaign launched in March put Pibor back in the spotlight after rights groups such as Amnesty International and Human Rights Watch documented a string of army abuses on civilians.

A rebellion by Yau Yau, a Murle himself, halted the disarmament process late last year, amid fears that disgruntled youth were ganging up.

As the dry season sets in, improving accessibility in South Sudan's largest and most underdeveloped state, there are fears that the fragile peace in Jonglei could break entirely.

"Unless community leaders and government authorities take decisive action to stop the violence, stability in the whole of Jonglei state will be at stake," Johnson said.