With reports of divisions within both warring camps, the fighting has fragmented the world's youngest country and multiple militia forces now pay no heed to either President Kiir or Vice President Machar.
A fresh ceasefire was announced this week by South Sudan's President Salva Kiir and Vice President Riek Machar after five days of violence that claimed around 300 lives in capital Juba.
The latest fighting broke out on the eve of July 9, which is the fifth anniversary of the country's independence from Sudan.
It has raised fears of a relapse of a civil war.
Who is fighting who
The rivalry between Kiir and Machar, both on the battlefield and in politics, is not new. Both men fought against each other and rose to power during Sudan's 1983-2005 civil war between north and south.
Civil war broke out again in South Sudan in December 2013 after Kiir sacked Machar as vice president for allegedly plotting a coup.
Following his dismissal, Machar had claimed that he planned to head the Sudan People's Liberation Movement (SPLM), which led the country to secede from Khartoum after fighting one of Africa's longest civil wars.
Machar is currently heading the Sudan People's Liberation Army in Opposition (SPLA-IO). Many elected members of Sudan's parliament, belonging to Kiir's SPLA, had defected to Machar's opposition and were later expelled from the House.
In May 2015 and during the civil war, South Sudan's 302 member parliament legislated to amend the country's transitional 2011 constitution to extend the presidential and parliamentary term until July 9, 2018, with 264 members voting in favour and a handful opposing it.
Kiir took over the reign of the Sudan People's Liberation Army (SPLA) following the death of his predecessor John Garang, who was killed in a helicopter crash in 2005.
Garang was a charismatic leader who launched the Sudan People's Liberation Movement (SPLM) in 1983 to rebel against the government in Khartoum.
Sudan was a British colony which gained independence in January 1956. It was divided between the Muslim-majority north and the Christian and animist dominated South.
The fighting between the forces loyal to the government in Khartoum and the Christian and animist southern rebels killed nearly two million people and displaced twice as many before a 2005 peace deal.
Garang also played a key role in securing the peace deal with Sudanese President Omar Hassan al-Bashir's National Congress Party.
The deal provided for north and south to share wealth and power, for national democratic elections in April 2010 and a southern referendum in 2011 on independence.
Garang was killed while travelling on the Ugandan presidential helicopter, just weeks after his triumphant return to Khartoum to join Sudan's government as first vice president under the deal to end the 22-year-long fighting.
Kiir, who is known as a no-nonsense man, won the April 2010 elections against his only rival and former ally Lam Akol with almost 93 percent of the vote.
He lead the then semi-autonomous region to independence after a referendum.
Kiir also commands great respect across the country for his role in uniting the often squabbling tribes and militias which make up the elite.
Of late he has been facing criticism from within his party as well as donors and common citizens for failing to deliver on the dividends of "peace" development.
Machar on the other hand, best known for his broad smile and marriage to the late glamorous British aid worker Emma McCune, is a controversial character in South Sudan.
He sided with Khartoum during the civil war, which led to the split and subsequent murderous violence between the rebel southerners.
Machar is also accused of being responsible for an inter-tribal massacre of thousands in Bor in 1991.
Machar is respected among his Nuer, one of the biggest tribes after Kiir's dominant Dinka.
He is well-spoken and holds a PhD degree from the US university of Bradford whereas Kiir spent much of his life in the bush.
End of civil war
The two-year war ended in the world's youngest nation after a peace deal was signed by the two leaders in August 2015.
The deal was brokered by the regional eight-nation IGAD bloc, along with the UN, the African Union, China, Britain, Norway and the United State. The country also faced UN sanctions if a deal could not have been reached.
Political wrangling over the nitty-gritty of the peace deal and sporadic violence ensued after Machar returned to Juba in April and retook the post of vice president.
Some analysts say that hardliners on both sides never supported the August 2015 peace deal as they want to end the conflict through military means.
Kiir's long-time ally and army chief Paul Malong, who is a top general and powerful politician commanding an ethnic Dinka militia opposes power-sharing with Machar.
Machar's authority over his forces is also questionable, particularly with regard to his command over ethnic militia such as the so-called ‘White Army'.
The fearsome militia of cattle-raiding youth pay little heed to anyone but their direct leaders.
UN experts have held both Kiir and Machar responsible for violence during the civil war which claimed the lives of thousands of people.
The United Nations Human Rights report published earlier this year had expressed grave concern over the encouragement of fighters from pro-government militia which fight alongside the Sudan People's Liberation Army (SPLA) to rape women as a mode of compensation.
Grotesque rights violations, including burning children, who were suspected of supporting the opposition, to death could amount to war crimes, said the report on the world's youngest country.
South Sudan still faces the issues of stabilising its security and economic situation. Oil production, which the economy heavily relies on, has plunged drastically
The 12.3 million-strong country is facing a record-breaking hunger crises this year with almost 5.8 million people facing food shortage.
Crisis-level food insecurity is faced by around 2.8 million, according to the latest figures issued by the United Nation's Food and Agriculture Organisation.
Unlike many of its drought-prone neighbours, the country which is almost the size of France is incredibly fertile and more than 90 percent of its land could be farmed.
But less than 5 percent of land was cultivated in 2014 because of the two years of civil war and heavy rains, FAO said.
Key aid agencies still struggle to stem the humanitarian crisis in the devastated nation
"Out of the $7.5 million which the World Health Organisation needs for health interventions in South Sudan, only $4.3 million had been received thus far. The health cluster as a whole was only 28 per cent funded", stated Tarik Jašarevic, spokesperson for the World Health Organization.
Moreover, around 2.5 million people have been internally displaced and more than 50,000 have been killed after civil war broke out in 2013.
According to the United Nations, an estimated 221,000 South Sudanese refugees have fled to Sudan since December 2013 due to armed conflict and food insecurity.
The latest fighting has forced 36,000 more people to seek refuge at the UN base at Juba, the UN added.
The latest clash
Though there are no confirmed reports, the latest conflict was apparently sparked on Thursday when Kiir's forces stopped and demanded to search vehicles with Machar's troops.
As the situation escalated, tanks and gunship helicopters were deployed with artillery and mortar echoing in parts of the city intensifying the battle which has broadly been fought along ethnic lines, pitting Kiir, an ethnic Dinka, against Machar, a Nuer.
A UN camp housing people previously uprooted by the war had been caught in the crossfire killing a Chinese UN peacekeeper and wounding several civilians, Chinese and Rwandan peacekeepers.
Japan's UN ambassador Koro Bessho said the Security Council could decide to boost the strength of the nearly 13,500-strong UN force.
The US State Department on Sunday had also demanded an immediate end to the fighting in South Sudan and ordered the departure of non-emergency personnel from the US Embassy in Juba.
The UN Security Council also demanded on Sunday that President Salva Kiir and his Vice President Riek Machar "do their utmost to control their respective forces, urgently end the fighting and prevent the spread of violence".
UN Secretary General Ban Ki-moon said he was "shocked and appalled" at the violence in the world's youngest nation.
The UN said that the latest clash had forced thousands of people to seek refuge. Apart from fleeing to UN bases, other citizens had gathered in churches and schools to seek shelter from the fighting.
Is there a solution?
Even though both Kiir and Machar have called for a ceasefire, there are doubts over how much control the two leaders have over their forces.
Reminiscent of the 2013 civil war, the latest reports have said that Vice President Machar has moved, along with his troops, outside of Juba.
"We had to move away from our base (in Juba) to avoid further confrontation," Machar's spokesman James Gatdet Dak in Nairobi told Reuters. "He is not returning to the bush, nor is he organising for war."
The spokesman also called for an outside force to be deployed to act as a "buffer" between the two warring sides.
UN chief Ban Ki-moon on Monday also called for the Security Council to impose an arms embargo on South Sudan.
The move was backed by the United States, France, Britain, Angola and other council members including Russia.
Earlier in January, Russia had said it was against an arms embargo.
But on Tuesday Russia's Ambassador to the UN, Vitaly Churkin said, "We're not completely opposed ... it's definitely a very difficult situation, so together with other members of the Security Council we are thinking about what can be done."
Though he added that he was wary of whether it would actually achieve anything.