New fighting began after a unit of loyalist forces clashed with the bodyguards of former rebel leader and current vice president Riek Machar.
The true nature of the conflict is said to reflect the ethnic division of South Sudan: Kiir's supporters are largely Dinka, and Machar's followers mostly Nuer, tribes with a long history of sectarian disagreement, primarily over cattle ranching and water rights.
There were eight deaths and 67 injuries at the United Nations base Sunday and Ban singled out the killing of two Chinese peacekeepers for condemnation.
UN Secretary-General Ban Ki-moon responded to the latest situation, calling for a blanket arms embargo on South Sudan, sanctions on the leaders of both sides in the civil war, and "fortifying UNMISS".
"We are committed to implementing the peace agreement we signed and we have a transitional government that is performing its missions despite the tough conditions", said Makuei.
South Sudan's President Salva Kiir ordered a unilateral ceasefire Monday. He blamed the opposition forces for lacking control over their forces.
"I personally don't think they command the respect of their soldiers, or that they command the trust of the people of South Sudan".
South Sudan's oil revenue has collapsed and the price of sorghum - the staple cereal crop - has increased by 400 percent this year.
The transitional administration "is now dead", said Alex de Waal, executive director of the World Peace Foundation at Tufts University in MA. "The idea of stationing two armies, still hostile, within or on the edges of Juba, and entrusting the security of the city to them, has been shown to be profoundly flawed".
Heavy fighting between the rival factions erupted again on Monday.
South Sudan's government said on Friday alone at least 271 people were killed.
Numerous thousands displaced by the fighting are seeking shelter at two U.N. bases, a World Food Program compound and other areas, said Matilda Moyo, a spokeswoman at the U.N. Office for the Coordination of Humanitarian Affairs.
Some 30,000 Juba residents were sheltering in the United Nations base in the Jebel district, where much of the fighting had raged since Friday, when 272 people, including 33 civilians, were killed.
"We, the leaders of the church in South Sudan, are extremely disturbed about the fatal shootings" in Juba, said the church leaders' statement. Several other peacekeepers have been injured, as well as a number of civilians who have been caught in crossfire. "But in a way, it's a situation that is not that surprising when you look at the history of South Sudan, not just because it's born out of war, but it was very clear even after signing the peace agreement in 2005 with the former enemy in the north in the Republic of Sudan that this was going to be a very, very hard process".