JUBA: Renewed gunfights broke out across South Sudan's capital between forces loyal to the president and those of the vice president, officials said Sunday, raising fears that the country is returning to civil war.

A U.N. camp for displaced people has been hit by the violence, causing many casualties, according to witnesses.

Government forces attacked a rebel base in the Jebel area of the capital Sunday morning, said William Gatjiath Deng, a spokesman for the rebel forces.

"Three helicopter gunships have just come now and bombed our side," he said.

Government spokesmen could not be reached for comment.

Gunfire from heavy weapons was hitting a U.N. camp for displaced people, said a resident of the camp, who insisted on anonymity, for fear of being targeted.

"It's just the heavy guns now going on," he said. "It started at 8 a.m. It's still even now we are in danger because there are some big bullets from these big weapons. They are landing inside the U.N.camp."

Sunday's fighting is a resumption of the conflict on Friday in which more than 100 people died. A precarious calm was restored on Saturday— the day South Sudan was to celebrate its fifth independence day — that was shattered by heavy gunfire Sunday.

South Sudan is trying to emerge from a two-year civil war caused by political rivalry between Vice President Riek Machar and President Salva Kiir.

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.

A similar skirmish in December 2013 sparked of the civil war that killed tens of thousands of people.