The United States has said an outbreak of violence that has left hundreds dead in South Sudan "must stop".
Anyone impeding efforts to end the fighting would be held fully accountable, the White House warned.
The UN has called for an immediate arms embargo, as well as attack helicopters to strengthen its peacekeeping force.
Days of fighting between forces loyal to President Salva Kiir and Vice-President Riek Machar in the capital, Juba, has left hundreds dead.
A ceasefire was called on Monday but it is not clear whether the violence has subsided. Heavy gunfire continued to be heard in the capital after President Kiir's ceasefire order came into effect at 1800 local time (1500 GMT) on Monday.
Two Chinese UN peacekeepers and one South Sudanese UN worker are among the hundreds of dead.
The US's National Security Adviser, Susan Rice, said: "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."
Peace deal shredded
South Sudan became independent from Sudan in 2011 but its short history has been marred by years of civil war.
The BBC's James Copnall says the latest clashes have traumatised Juba and shredded a peace deal between Mr Kiir and Mr Machar, agreed last August.
He said there were also doubts over how far both men were in command of their forces. Both leaders had called for calm last Friday, but the fighting carried on.
Thousands of people have had to leave their homes.
Hundreds of them have sought refuge in UN premises, including Jebel and Tomping bases in Juba which were caught in crossfire on Monday.
UN Secretary-General Ban Ki-moon said: "Yet again, the leaders of South Sudan have failed their people. Rarely has a country's conduct squandered so much promise so quickly."
Mr Ban said an "immediate" arms embargo should be imposed on South Sudan, political and military leaders blocking the peace deal should be sanctioned and the UN mission to the country should be fortified.
He also called for military equipment "to fulfil our mandate to protect civilians" and said countries with troops in South Sudan should "stand their ground" rather than withdraw in the face of the violence.
On Monday afternoon, an army spokesman told the BBC that soldiers loyal to Mr Kiir had been ordered back to barracks.
Any who resisted the order, by loitering or looting, would be arrested, he said.
Why has fighting resumed?
It seems a disagreement at a checkpoint between rival soldiers led to a shootout on Thursday night in which five soldiers died. This quickly escalated into serious fighting from Friday onwards. Tensions have been high since April, when Mr Machar returned to Juba under a peace deal following a two-year civil war. He took a 1,300-strong protection force with him and they were supposed to start joint patrols with forces loyal to President Kiir. But a lack of trust between the two sides means the patrols have not begun.
Will it become a new civil war?
There are concerns that what we are seeing is a repeat of what happened in December 2013. The two-year civil war started after clashes between rival soldiers in Juba and degenerated into nationwide conflict in which tens of thousands died. The war was fought broadly between South Sudan's biggest ethnic groups - the Dinka, led by Mr Kiir, and the Nuer, under Mr Machar.
What can the international community do?
The international community played a major role in the creation of South Sudan and has tried to exercise some influence since independence in 2011. The UN and US have called for an immediate end to fighting, a call echoed by the East African regional group which brokered the recent peace deal.
