"We need to do everything to prevent such a cycle of violence between the communities in South Sudan," Ms. Johnson said, stressing that those responsible need to be held to account.
"I condemn in the strongest possible terms the atrocities committed against innocent civilians of different communities by elements from both sides," she added.
The UN Mission yesterday voiced grave concern over mounting evidence of gross human rights abuses in the strife-torn country, including extra-judicial killings of civilians and captured soldiers, massive displacements and arbitrary detentions, often with ethnic undertones.
UNMISS cited the discovery of large numbers of bodies in Juba, the capital, and the Upper Nile and Jonglei state capitals of Malakal and Bor.
UNMISS has been collecting information every day since the crisis began and pledged to continue to investigate all reports of serious human rights violations, and to collect evidence and eyewitness testimony in order to document such allegations.
It reminded all parties of their obligation to protect civilians and act in accordance with human rights and humanitarian law, and called on key leaders to send strong public messages to their respective constituencies insisting that the violence must stop, and that anyone disobeying these orders will be punished severely.
Ms. Johnson welcomed the African Union's Peace and Security Council's decision to establish a commission to investigate human rights violations and other abuses and recommend ways and means to ensure accountability, reconciliation and healing among all communities.
Meanwhile, UN agencies and humanitarian partners, supported by UN blue helmets working to protect camps, are working to provide families with water, shelter, food and other emergency relief.
Aid agencies will need $166 million to respond to immediate needs until March, Ms. Johnson reminded the international community.