It comes as South Sudan's president and vice president have ordered their rival forces to cease hostilities after days of fighting that have left hundreds of people dead.
The renewed violence has threatened to plunge the world's youngest country back into civil war.
Fighting started four days ago in the capital Juba between followers of President Salva Kiir and Vice President Riek Machar.
The violence broke out as South Sudan prepared to mark five years of independence from Sudan last Saturday.
Presidential spokesman Ateny Wek Ateny says both men have now spoken by phone and have called for a unilateral ceasefire.
"With effect from 11th July, 2016, at 6pm (Juba time), all the commanders of these forces are hereby directed to cease any hostility and abide by this order, control their forces and protect the civil population and their properties."
The fighting has raised fears of a return to the civil war that erupted in December 2013.
That conflict killed tens of thousands of people and forced more than 2.5 million people from their homes.
United Nations secretary general Ban Ki-moon has spoken clearly about who he believes is responsible for this latest surge in violence.
"Yet again, the leaders of South Sudan have failed their people. Rarely has a country squandered so much promise so quickly. What kind of leadership is it that resorts to deadly weapons and identity politics? Time and again, failed leadership."
The UN Security Council has demanded Mr Kiir and Mr Machar control their forces and end the fighting.
The UN mission in South Sudan, known as UNMISS, says its bases in Juba were caught in the crossfire between the two sides.
It says two Chinese peacekeepers were killed in that incident.
The head of the mission, Ellen Margrethe Loj, has condemned the attacks.
"The United Nations condemns, in the strongest terms possible, the use of heavy weapons, including rockets fired from attack helicopters, in the immediate vicinity of UNMISS protection of civilian sites and the UN compound in Jebel. This constitutes a serious violation of international law."
Ban Ki-moon has urged the Security Council to impose tough restrictions on South Sudan.
"Impose an immediate arms embargo on South Sudan. Second, enact additional targeted sanctions on leaders and commanders blocking the implementation of the agreement. Third, fortify the UN mission in South Sudan, UNMISS. We desperately need attack helicopters and other material to fulfil our mandate to protect civilians. I also urge all countries contributing to UNMISS to stand their ground, while withdrawals would send precisely the wrong signal, in South Sudan and across the world. Any withdrawals would send precisely the wrong signal in South Sudan and across the world."
Marika Shameras is a research fellow with the London-based Overseas Development Institute.
Speaking to Al Jazeera, Ms Shameras says the unrest in South Sudan does not come as a surprise.
"What has caught people by surprise is really the strength of some of the fighting, and this is what we saw the last time when fighting broke out in December 2013. Just the speed and strength of fighting was really quite surprising, even to those who were very, very skeptical about the chances of South Sudan remaining entirely peaceful. 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 difficult process."
Ms Shameras says South Sudan faced many challenges in the time before it seceded from the north.
"In many ways, what we see now is also just a coming real, or manifestation, of a lot of problems that were hidden by a lot of euphoria and enthusiasm for a very long time. It is very difficult to build a nation-state out of a history of war, a nation-state where, really, it's very hard to reach some areas because they have hardly any roads going there. It's very, very difficult to bring all this together, and I think, for a very long time, both the South Sudanese as well as the international community were driven by a make-believe, or just trying really hard to make it work, because there was a great dream of a South Sudanese nation."
