On Thursday, armed herders killed 21 civilians in a reprisal attack on a rival cattle camp in Kajo-Keji County of Central Equatoria, the county commissioner's office said.
"The commissioner of Kajo-Keji County condemns in the strongest terms possible the attack on the cattle camp and the massacre of the innocent civilians in the barbaric act of revenge", its statement issued on Thursday said.
Reuters reported that the death toll could be at least 27.
The killings take place ahead of Pope Francis visit Friday to South Sudan.
Justin Welby, the Archbishop of Canterbury who will also be in South Sudan, said he was "horrified" by the attack on the eve of his visit.
"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 posted on Twitter on Thursday.
The church leaders are seeking to promote reconciliation and forgiveness in a predominantly Christian country still burdened by chronic armed violence in the aftermath of a civil war.
Equatorial State, in the country's north, has witnessed some of the most ferocious armed violence in South Sudan in recent months, with thousands of civilians seeking protection on UN bases.
"With the historic visit of His Holiness Pope Francis, the Archbishop of Canterbury and the Moderator of the Church of Scotland to South Sudan expected to take place this week, UNMISS appeals to national and community leaders to exercise restraint and commit to peace and dialogue," it said in a statement.
This report was sourced from Agence France Presse and Reuters.