Ban Ki Moon, meanwhile, called for the Security Council to re-enforce the embattle peacekeeping mission and to impose a full arms embargo on the country.
South Sudan's president ordered a ceasefire on Monday after a new day of heavy fighting in the capital Juba that sent thousands of people fleeing and threatened a return to civil war.
As fighting continues in South Sudan, tens of thousands of civilians have taken refuge inside a United Nations base, The Associated Press reports.
Two U.N. peacekeepers from China were killed at the base Sunday night, according to Chinese state media.
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.
In response to the ongoing violence, the U.S. State Department on Sunday ordered the departure of non-emergency personnel from the U.S. Embassy in Juba, according to a statement from spokesperson Kirby.
The fighting that began Thursday and lasted through the weekend caused some residents to question whether their hard-won independence was worth it.
The global community must also impose targeted sanctions against civilian and military officials reasonably suspected of responsibility for crimes under worldwide law or human rights violations.
In December 2013, soldiers from Kiir's Dinka ethnic group tried to disarm Nuer soldiers perceived to be loyal to Machar. Thousands more fled to the camp in the current fighting.
"It is not a small matter, it is about trying to put aside the entrenched political differences but especially to also to try and put their eyes on the people of South Sudan".
Malual said he had been displaced by the fighting to a suburb of Juba, suffering from food and water shortages.
South Sudan split from Sudan in 2011 after years of fighting.
A major fear is that fighting will spread to other areas in South Sudan, IGAD Chairman Tedros Adhanom told reporters in Nairobi.
"Government officials have repeatedly accused the civilians inside the United Nations bases of being rebels or rebel supporters".
The compound "is designed for about 100 people and it's got something like 3,000 in it right now", McDonough said.
"(Our staff) said they've tried to leave their neighbourhood because they don't feel safe and they are taking cover under their beds with their children", Concern Worldwide's country director Feargal O'Connell said via Skype from Juba. No injuries have been reported among Japanese Self-Defense Forces troops on standby in the area, Saito said.
The conflict killed thousands of people, forced more than 2.5 million people from their homes and left nearly half the population of 11 million people struggling to find food.
A peace deal was signed in August 2015, but experts say failure to implement swiftly key elements, such as the re-integration and demobilization of combatants, have allowed tension to fester even as the country turned five-years-old on July 09.