South Sudan's President Salva Kiir on Sunday agreed to the deployment of a regional protection force to beef up the UN's large UN peacekeeping mission in the war-scarred nation after initially opposing it as a breach of national sovereignty.

The announcement came after the leader of the world's youngest nation met with ambassadors of the UN Security Council at the bullet-scarred presidential palace in the capital Juba.

"The transitional government of national unity gives its consent for the deployment of the regional force," said a joint statement by the UN and the government, which was read out to the media by South Sudanese Cabinet Affairs Minister Martin Elia Lomoro.

"We will design the modalities," Lomoro said, without elaborating.

The presidential palace was the scene of clashes on July 8 between President Kiir's guards and troops loyal to his former deputy Riek Machar, shattering a fragile truce that has been breached several times.

Kiir showed the ambassadors from the council's 15 member states bullet marks in the heart of the building as well as shattered window panes. He said Machar had wanted to assassinate him that day but claimed he had helped his rival to escape to safety.

Following the violence, the UN Security Council authorised the deployment of an additional 4,000 troops from East Africa with a stronger mandate than the 13,000-strong UN peacekeeping mission UNMISS.

The UN officials arrived on Friday in a bid to secure Kiir's agreement to the extra troops.

- Words into action -
UNMISS has faced considerable criticism over its failure to protect civilians during the July violence, which included the rape of civilians sheltered just outside its camps.

Kiir had opposed the deployment of additional troops, initially touted as an "intervention force", as breaching national sovereignty.

Minister Lomoro also underscored that the government committed "to permit free movement to UNMISS in conformity with its mandate" and "improve humanitarian access, including by providing assistance by eliminating illegal check points."

Samantha Power, the US envoy to the Security Council, hailed the move but said it was now important to start translating words into action.

"What we need to do now is move from those very important high-level commitments into working up the modalities in an operational way," Power said.

"UNMISS has an impartial mandate to protect civilians, no matter who they are, no matter where they are. The number one obstacle for (the peacekeepers) fulfilling their mandate to this point has been the severe restrictions on their movements."

Earlier Sunday, the UN team met with displaced people in the northern town of Wau, the scene of bitter fighting in recent months.

Catherine Atanasyo, a local area chief from the south of Wau now living in the camp, said lawlessness was rampant.

"Looting is going on in town. We don't know when we'll be going home," she told them.

Speaking to AFP later, Atanasyo said the regional force was badly needed as members of the president's Dinka tribe were raping and killing people if they went into Wau.

"We cannot leave this camp without security and that security can be provided by the regional force," she said.

South Sudan descended into war in December 2013 after Kiir accused his former deputy Machar of plotting a coup.

During the fighting in July, Machar, who had been persuaded to return to Juba to join a national unity government agreed under a peace deal, fled the country and is now in Khartoum, having been replaced by Taban Deng Gai in Juba.

Aside from the tens of thousands of people killed, the United Nations has reported shocking levels of brutality including gang-rapes and the wholesale burning of villages.

An estimated 16,000 children have been recruited by armed groups and the national army in the conflict, and 2.5 million people have been driven from their homes.