The UN Security Council has called on warring factions in South Sudan to immediately end the recent fighting and prevent the spread of violence.
In a unanimous statement, the council condemned the fighting “in the strongest terms” and expressed “particular shock and outrage” at attacks on UN sites.
It also called for additional peacekeepers as a response.
Hundreds are reported killed in clashes between rival groups since Friday.
Forces loyal to Vice-President Riek Machar say government troops supporting President Salva Kiir attacked their positions in the capital, Juba.
A spokesman for Mr Machar told the BBC on Sunday that the country was “back to war” – but Information Minister Michael Makuei Lueth described reports of war as “dishonest”.
Calm was restored on Saturday, but fighting resumed on Sunday morning.
The UN mission said hundreds of people had sought shelter in its compounds.
UN officials said a Chinese peacekeeper was killed and several Chinese and Rwandan troops injured.
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.
The violence has raised fears of renewed instability, with a 2015 peace deal failing to quell unrest.
Friday’s exchanges were apparently sparked by a shootout between President Kiir’s and Mr Machar’s bodyguards. At least 150 died in the clashes.
The two men met at the presidential palace the same day, and issued a call for calm.
Calm was apparently restored on Saturday but heavy gunfire was reported on Sunday near a military barracks occupied by troops loyal to Mr Machar.