Juba: South Sudan's president and vice-president ordered their loyalists to cease hostilities on Monday after days of fighting threatened to plunge the country back into civil war and bring further instability to one of the most impoverished regions of Africa.

Fighting erupted four days ago in the capital Juba between followers of President Salva Kiir and Riek Machar, the former rebel leader who was reinstated as vice -president under a deal to end a two-year civil war.

South Sudanese Vice-President Riek Machar, left, and President Salva Kiir, centre, call for calm at the presidential palace in Juba. Photo: AP

The violence, which has killed hundreds of people, broke out as the world's newest nation prepared to mark five years of independence from Sudan on July 9.

United Nations Secretary-General Ban Ki-moon on Monday urged the UN Security Council to impose an arms embargo on South Sudan and sanction leaders and commanders who blocked the implementation of a peace deal.

Presidential spokesman Ateny Wek Ateny said Mr Kiir and Dr Machar had spoken by phone on Monday, a day which saw tanks and helicopters involved in some of the fiercest clashes yet.

Troops from the rebel Sudan People's Liberation Army In Opposition (SPLA-IO) at a camp near the Jebel mountains on the outskirts of Juba, South Sudan. Photo: Kate Geraghty

"All the commanders of (Mr Kiir's) forces are directed to cease any hostility and abide by the order and control their forces," Mr Ateny said. "President Salva Kiir is determined to carry on his partnership with Riek Machar."

Dr Machar responded by ordering his troops to stand down.

"The president has declared a unilateral ceasefire, I want to reciprocate the declaration of unilateral ceasefire," he told independent Eye Radio.

At least 3000 displaced civilians seek shelter at the UN compound in the Tomping area of Juba. Photo: UNMISS via AP

Much is unclear, however, about the latest violence in Juba, including what the objective of either side has been and how much control the two leaders have over their forces.

The UN Mission in South Sudan (UNMISS) expressed its "outrage" after its bases in Juba were caught in the crossfire between the two sides and two Chinese peacekeepers were killed.

Irene Scott, a former ABC journalist working as a radio producer for Internews, is sheltering in her Juba accommodation's "safe room" during bouts of fighting.

Footage released by China's CCTV shows Chinese UN peacekeepers tending to a wounded colleague after an attack on an armoured personal carrier in Jebel in South Sudan. Two Chinese peacekeepers were reported killed in the attacks. Photo: CCTV via AP

"A lot of the fighting has been around where we live and where the [UNMISS] base is," she said, describing "heavy artillery and helicopters flying overhead" punctuated by "surreal moments of absolute quiet".

Working in Juba's UN Protection of Civilians site, Scott is "very nervous" about her local staff living there. 

"I really feel for them being stuck in the middle of a war they prefer was not happening," she said.

Black smoke rising above the South Sudanese capital Juba. Photo: AP

The US State Department said it was carrying out an "ordered departure" of its staff from South Sudan.

Australian's Department of Foreign Affairs and Trade did not respond to questions about possible flight evacuations for Australian citizens in South Sudan.

A statement released by the Foreign Minister's office on Tuesday said: "Australians in Juba should remain in a safe location, avoid movement on the streets and monitor the media. The Australian government's ability to provide consular services in South Sudan is extremely limited."

The Australian Defence Force has a contingent of up to 25 personnel in the country, most of whom are based in Juba, and says it is "adjusting force protection measures for our personnel where required".

A rebel South Sudanese soldier returns to Juba as part of the April peace agreement that saw Riek Machar reinstated as vice-president but is now in peril. Photo: Kate Geraghty

The civil war erupted in December 2013 and has run broadly along ethnic lines, pitting Mr Kiir, an ethnic Dinka, against Dr Machar, a Nuer.

The conflict killed thousands, forced more than 2.5 million people from their homes and left almost half the population of 11 million people struggling to find food. Oil production, by far the largest source of government revenue, has plummeted.

A new flare-up risks driving yet more people to refugee camps in neighbouring countries in the central African region, which is already plagued by myriad woes.

Central African Republic is riven by conflict, the eastern region of the Democratic Republic of Congo is contending with a patchwork of militias and rebel groups, and Burundi is embroiled in a violent political crisis.

The United States condemned the violence and said it would hold those who commit atrocities or impede efforts to stop the fighting "fully accountable".

"We call on those fighting to return to their barracks. This senseless and inexcusable violence, undertaken by those who yet again are putting self-interest above the well-being of their country and people, puts at risk everything the South Sudanese people have aspired to over the past five years," White House National Security Adviser Susan Rice said.

Mr Kiir and Dr Machar have long been rivals, both in politics and on the battlefield. Civil war broke out in 2013 a few months after Mr Kiir sacked Dr Machar as his deputy.

Experts say the failure to swiftly implement important elements of the deal, such as integrating and demobilising their forces, has allowed tension to fester and risked igniting a new conflict.

There has been no official death toll from the recent flare-up but at least five soldiers died on Thursday and a Health Ministry source said 272 people, including 33 civilians, were killed on Friday. Sunday and Monday's fighting was more fierce.

UNMISS said gunfire had erupted on Monday around the UN headquarters in the Jebel area of Juba and also around a base near the airport. It said UN sites had been hit in exchanges of fire, killing eight and injuring 67. 

Highlighting lawlessness on Juba's streets, one resident said he saw police officers trying to loot a shop in his neighbourhood by shooting off the padlocks and firing in the air to scare away people, echoing similar witness reports on Sunday.

The African regional grouping IGAD echoed Mr Ban's call to beef up UNMISS's mandate, calling for it be given an enforcement role similar to a UN-backed intervention brigade working in eastern Congo.

Through the civil war, however, world powers and regional states struggled to find leverage over the warring factions, despite US and European sanctions on some military leaders and African threats of punitive actions.

Reuters, with Colin Cosier