Twenty-seven people have been killed in South Sudan in tit-for-tat violence involving cattle herders and militia fighters, a regional government official said, as the country prepares to welcome Pope Francis.

The pope is set to arrive in South Sudan on Friday from neighbouring Democratic Republic of Congo, hoping to jolt a peace process aimed at ending a decade of conflict fought mostly on ethnic lines that has killed hundreds of thousands of people.

Fighters from a rebel group on Thursday killed six people from a herding community in Central Equatoria state. Herders retaliated later in the day by killing 21 civilians in a nearby area, including five children and a pregnant woman, said Kajo-Keji county commissioner Phanuel Dumo.

Dumo accused the National Salvation Front (NAS) - one of a handful of anti-government militias operating in the country - of attacking the herders. NAS denied it was responsible.

Mayom Ateny Wai, Secretary General of the Bor community of herders, denied retaliating against civilians and said NAS was to blame for those deaths.

A peace deal signed in 2018 by the main parties to civil war from 2013-2018 has significantly reduced violence in South Sudan in recent years.

But lower-level clashes between rival communities regularly flare up. And violence remains rife in areas where rights to grazing areas, water, cultivation grounds and other resources are under dispute.

The Archbishop of Canterbury, who is accompanying the pope to South Sudan, said he was horrified by the latest killings.

"It is a story too often heard across South Sudan. I again appeal for a different way: for South Sudan to come together for a just peace," he said on Twitter.