At least 115 soldiers from South Sudan's rival factions were killed in gunbattles in the capital Juba, a military spokesman for the opposition said on Saturday, amid fears for a fragile peace process in a country still reeling from a two-year war.

Gunfire erupted on Friday evening near the state house where President Salva Kiir and vice president Riek Machar, former rivals, were meeting for talks.

Both men said they did not know what had triggered the latest fighting between their factions and called for calm.

William Gatjiath Deng, spokesman for Machar's military faction, said the fighting had occurred near the state house and in army barracks.

"In the morning we collected and counted 35 (dead) from the SPLM-IO (Machar's faction) and 80 people from the government forces," he said.

Deng said the death toll could rise on Machar's side "because there are some soldiers seriously wounded".

The government side had no immediate comment on the situation in Juba.

At least five soldiers were killed on Thursday in similar clashes between the two sides.

Africa's newest nation is emerging from a two-year civil war which started in December 2013 after Kiir sacked Machar as vice president.

The conflict was largely fought along ethnic lines with Kiir mostly drawing support from his Dinka tribe while Machar was backed by his Nuer tribe.

A peace agreement last August ended the war but Kiir and Machar have yet to integrate their forces, a key part of the agreement.

Thursday and Friday's gunfights were the first major outbreak of violence in Juba since Machar returned to the capital in April after his re-appointment as vice president.

A Reuters witness said on Saturday Juba was calm but tense with road blocks mounted on some streets. Heavy military vehicles could be seen patrolling and most businesses were shuttered.

"It seems as though things have certainly calmed from what they were last night. However the situation is still very, very tense," said Jeremiah Young, an aid worker with World Vision.

Young said there was a chance the security situation could "deteriorate very quickly due to the tensions within Juba and the surrounding areas."

U.N. Secretary-General Ban Ki-moon said on Friday the latest violence highlighted a lack of commitment to the peace process and urged the country's leaders to end the fighting, discipline military leaders and work together to implement the peace deal.-Reuters