Government forces and former rebels clashed Friday night outside the Presidential Palace in South Sudan's capital Juba, imperiling an internationally backed peace accord seeking to end more than two years of bloody civil war.
Clashes among rival factions left at least five government soldiers dead as the world's newest nation is set to mark the fifth anniversary of its independence.
Both President Salva Kiir and First Vice President Riek Machar, who the rebels are loyal to, have called on their forces to refrain from violence and urged peace.
Heavy gunfire and artillery could be heard and military helicopters have also been spotted hovering above the city.
Kiir said he had been meeting Machar and Second Vice President James Wani to discuss an incident Thursday night when five soldiers were killed in a skirmish in Gudele between the government's Sudan People's Liberation Army (SPLA) and Machar's Sudan People's Liberation Army-in-Opposition (SPLA-IO) forces.
"What is happening outside is something that we cannot explain to you," said Kiir. "We were discussing the situation of yesterday and then talking about what we can do in the implementation of the [peace] agreement and to build confidence among the forces and the civil population," Kiir said, as quoted by American-funded online Radio Tamazuj.
"This is a very unfortunate incident which none of us really knows what has happened," said Machar. "All we want to tell our public now is that they should remain calm, they should remain calm, this incident also will be controlled, and measures will be taken so that peace is restored even to the heart of the city itself."
Despite the forming of a unity government and power-sharing between Kiir and his former rival-turned-deputy Machar, violence has continued across the country, particularly among local militias driven by local agendas.
Tens of thousands of people have died and more than 2 million people uprooted from their homes in the two years civil war that started in December 2013.
In a statement released Friday night, UN Secretary-General Ban Ki-moon said he was "deeply alarmed" by the ongoing fighting, calling it "yet another illustration of the parties' lack of serious commitment to the peace process and … a new betrayal of the people of South Sudan."
He urged Kiir and Machar to "put an immediate end to the ongoing fighting, discipline the military leaders responsible for the violence, and finally work together as partners to implement" the peace agreement.