Story highlights U.N. Secretary-General Ban Ki-moon backs East African leaders' call for peace

Ban warns those responsible for violence and human rights abuse 'will be held accountable'

Doctors Without Borders also calls for end to violence

A growing chorus of international agencies is calling for South Sudan's warring factions to cease hostilities.

A day after East African leaders warned both the fledgling country and its rebel group to lay down their arms within four days, U.N. Secretary-General Ban Ki-moon threw his weight behind that ultimatum from the Intergovernmental Authority on Development (IGAD).

In a statement released Saturday, Ban said he lauded IGAD for "appointing a mediation team to work with the government of south Sudan and opposition in reaching a cease-fire, the release of the detainees and building toward a process of peaceful dialogue."

"The United Nations stands with the people of South Sudan and will continue to do everything within its means to protect civilians at risk and provide necessary humanitarian assistance," the U.N. statement continued. "All violence, attacks and human rights abuses must end immediately. The Secretary-General reminds those responsible that they will be held accountable. He calls on the government and all concerned to ensure the rights and security of civilians are protected."

It's unclear what the repercussions would be for either side if they refuse the demands.

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A communique issued Friday in Nairobi, Kenya, appears to throw IGAD's weight behind South Sudanese President Salva Kiir, whose government has said that it would come to the negotiating table without preconditions.

Kiir has accused rival politician Riek Machar, his former vice president, of trying to stage a coup-- a claim Machar has denied.

Kiir's Vice President, James Wani Igga, says Machar will make it difficult for both sides to meet IGAD's truce deadline.

"Dr. Riek has put obstacles to this genuine call by issuing preconditions: namely that the cease-fire or peace cannot be reached unless a negotiation is conducted, and added a number of obstacles," the South Sudanese Vice President said Saturday.

Kiir and Machar are longtime rivals from two different tribal clans. Kiir is from the Dinka tribe, Machar from the Neur.

Fighting broke out between Kiir's government and supporters of Machar on December 15 in the capital city of Juba. It quickly spread across the country, with reports of mass killings lent credence by mass graves.

Worried about the safety of medical staff and facilities, Doctors Without Borders is also asking both sides to alleviate "the volatile and rapidly evolving security situation."

The international aid group says it has provided medical care to approximately 40,000 people who have fled to two U.N. bases in Juba since the latest wave of violence began. The U.N. says more than 63,000 total civilians are now crowded onto its compounds across South Sudan.

Even though a curfew is in place in Juba, and though the capital has been relatively safe and stable compared to other parts of South Sudan for the past week or so, scared and beleaguered civilians have refused to leave the U.N. compounds.

On Friday, 72 peacekeepers arrived in Juba, according to the United Nations. It was the first group of an additional 5,500 peacekeepers approved by U.N. Security Council. The additional personnel will increase the total peacekeeping force in South Sudan to 12,500 soldiers and 1,323 police officers.