Following a day of calm on Saturday, fighting raged in several neighborhoods in Juba on Sunday as former rebels and government soldiers exchanged fire on the outskirts of the South Sudanese capital.
"Gunshots, heavily armed exchange UN House area once again; going on now since approx. 0825 (0525 UTC)," the UN Mission in South Sudan (UNMISS) said on Twitter.
The UN runs a camp in the area for people uprooted by the war, close to where both former rebels and government soldiers are camped.
Residents reportedly fled as violence escalated with the use of mortars, rocket-propelled grenades and "heavy ground assault weaponry."
Regional airline Kenya Airways suspended flights to Juba, citing the "uncertain security situation." The US Embassy in Juba warned its citizens to stay indoors.
Officials at President Salva Kiir's office put the death toll at 270 people since Friday.
On Sunday, Kiir called for an immediate ceasefire to end the conflict between the army and troops loyal to former rebel leader-turned-vice president Riek Machar.
A spokesman for Machar blamed government troops for the violence.
"Our forces have been attacked at Jebel base," said James Gatdet Dak, who claimed the attack had been repulsed. "We hope it will not escalate."
Machar's residence was attacked twice during Sunday's violence. Tanks and helicopter gunships were both reportedly used in the attacks.
UN Secretary-General Ban Ki-moon called on South Sudanese leaders to take "decisive action" to end the fighting.
"I am deeply frustrated that despite commitments by South Sudan's leaders, fighting has resume," Ban said in a statement. "This senseless violence is unacceptable and has the potential of reversing the progress made so far in the peace process."
Fears over surge in conflict
The outbreak of fighting on Sunday was the first since Friday when brief but heavy exchanges of fire left an estimated 150 soldiers dead on both sides, on the eve of the country's fifth anniversary of independence. Plummeting oil production and a resulting lack of money prompted South Sudan to cancel celebrations, however.
The recent surge in violence in the world's youngest country represents yet another blow to a shaky peace deal that has so far failed to end the civil war, which had broken out in December 2013, when Kiir accused Machar of plotting a coup.
The ensuing skirmish - similar to that seen at the weekend - sparked a civil war that results in tens of thousands of deaths and displaced more than 2 million people.
In a statement, the UN Security Council said the recent fighting showed a "lack of serious commitment" to the 2015 peace on the part of Kiir, Machar and their supporters.
The two leaders issued a joint call for calm after Friday's fighting, which began outside the presidential compound where Kiir and Machar were meeting and soon spread through the city.
ksb/sms (AFP, AP, Reuters)