Almost 300 people are reported to have been killed, including many civilians and Chinese peacekeepers, after the renewed fighting in Juba.
The clashes broke out between troops loyal to President Salva Kiir and soldiers who support Vice-President Riek Machar.
"We appeal to all warring factions to respect civilian lives, to respect wounded people, and to respect the humanitarian mission, and to respect the medical missions, the hospitals and healthposts," Juerg Eglin, from the International Committee of the Red Cross, told RFI.
"We fear that there's severe abuse, acts were committed that are way beyond standards of international humanitarian law and we call for restrain and respect. We do not have a full picture of what actually happened but we have some worrisome reports coming in."
Appeals to end fighting ignored
Despite several calls for fighting to end, the clashes are continued on Monday.
"We have two compounds in Juba, one is downtown, close to the international airport and the other is the UN house compound, our headquarters, in the Jebel area. So at approximately 9.00am this morning heavy gunfire could be heard in the close proximity of the UN house compound coming from the north-east direction," Shantal Persaud, the spokesperson for the UN Mission in South Sudan, told RFI.
"Simultaneously, we've heard reports from our other base that heavy fighting has also started again on the outer perimeter of our base there."
Already, according to the UN, up to 2,000 civilians are believed to have sought sanctuary in the several UN compounds in recent days.
That raises great concerns on the ground.
Analysts say too little has been done since the implementation of the peace deal last August.
"The so-called peace agreement has had a number of flaws, and perhaps central among them was a failure to really deal with any of the underline political issues but simply just try and bring everyone back together into Juba. That, inevitably, was going to lead to an increase of tension," Ewan Lawson, from the RUSI Institute in London, told RFI.
"The requirement of both parties to have robust security arragements for their leadership whilst Juba was, in theory at least, demilitarised, there were still going to be a large number of relatively well-armed troops in Juba, some of whom, not so long ago, would directly fight each other."
Observers say it is clear that the peace deal between the two main factions in the country is only holding “by a thread”.
Leaders seem to have lost control
But it also sounds as if the leaders of rival factions, the president and the vice-president, have no control over what's going on in the streets of Juba - not surprising considering the very tense atmosphere in the country in the past months.
"In terms of security, the country is a mess, the UN mission seems relativey powerless to do anything, despite having a pretty robust mandate and until such time some sort of stability can be brought, it's difficult to see how the political leaders can find the political space to be able to have meaningful conversations," Lawson explains.
"That said, though, those political leaders have been part of the problem. Those political leaders have contributed to quite widespread corruption, quite widespread frustration amongst the people."
The UN Security Council has urged both sides to end fighting and called for more peacekeepers after their meeting on Sunday.