JUBA, South Sudan — Fighting between competing factions of South Sudan’s government in its capital left the world’s youngest country teetering on Saturday, the fifth anniversary of its independence. The government reported dozens of casualties, and there were unconfirmed reports that up to 150 people had been killed.

Outbreaks of shooting on Friday night between forces loyal to the country’s president, Salva Kiir, and forces loyal to its vice president, Riek Machar, surprised both leaders, said Ateny Wek Ateny, a spokesman for Mr. Kiir. Heavy gunfire was exchanged across neighborhoods during the night, including outside the presidential palace, where the two leaders were meeting.

…

“This is the most perilous moment for South Sudan since the violence of December 2013,” said John Ryle, the director of the Rift Valley Institute and an analyst on the two Sudans.

The shooting that erupted as both leaders were meeting inside the presidential palace, Mr. Ryle said, “reveals the uncertain control they exercise over their own security forces — and the extreme fragility of the peace accord.”