Renewed fighting has broken out in South Sudan between forces loyal to the president and vice-president.
A reporter in the capital, Juba, told the BBC gunfire and large explosions could be heard all over the city. He said heavy artillery was being used.
More than 200 people are reported to have died in clashes since Friday.
The latest violence came hours after the UN Security Council called on the warring factions to immediately stop the fighting.
In a unanimous statement, the council condemned the violence “in the strongest terms” and expressed “particular shock and outrage” at attacks on UN sites. It also called for additional peacekeepers to be sent to South Sudan.
Residents of Juba have told the BBC they are staying indoors amid the sound of gunfire from the streets.
“Soldiers are now looting some property,” charity worker Ladu David Morris said. “They pretend to be patrolling but once they notice that no people are inside they break in.”
Chinese media say two Chinese UN peacekeepers have now died in Juba. Several other peacekeepers have been injured, as well as a number of civilians who have been caught in crossfire.
The latest round of violence erupted when troops loyal to President Salva Kiir and first Vice-President Riek Machar began shooting at each other in the streets of Juba.
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.
There are concerns that what we are seeing is a repeat of what happened in December 2013. The two-year civil war started then 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.
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.
On Monday, there were reports of tanks on the streets of Juba and clashes close to the airport and UN camps sheltering civilians.
The US embassy warned of “serious fighting” taking place.
A BBC correspondent in the Kenyan capital, Nairobi, said it was not clear if Mr Kiir and Mr Machar remained in control of their forces.
A UN spokeswoman in Juba, Shantal Persaud, said fighting over the past few days had caused hundreds of internally displaced people to take refuge in UN premises.
She said both South Sudanese leaders were responsible for implementing last year’s peace agreement, which included a permanent ceasefire and the deployment of forces away from Juba.
Information Minister Michael Makuei told the BBC that the situation in the city was “under full control” and civilians who had fled should return to their homes.
Mr Machar’s military spokesman, Col William Gatjiath, accused officials loyal to the president of lying, and said there had been at least 10 hours of clashes on Sunday.
“The situation in South Sudan is uncontrollable because Salva Kiir and his followers are not ready to follow the peace agreement,” he said.
The chairman of Igad, the East African regional bloc that has overseen peace talks in South Sudan, warned on Monday that the situation risked exposing millions of civilians “to senseless violence and the whole country to interminable chaos”.
In a statement on Sunday, the US state department said it strongly condemned the latest outbreak of fighting in Juba.
Spokesman John Kirby said Washington had ordered the departure of non-emergency personnel from the US embassy in Juba.