The attacks happened amid reports of increased local conflicts ahead of the scheduled elections late this year.
A civil war from 2013-18 caused hundreds of thousands of deaths and although the main belligerents have since been at peace, clashes continue among armed groups.
Activists believe the recent rise in violence is at least partly connected to the elections to choose leaders to succeed the present transitional government.
In the latest incident on Sunday, youths attacked Ajwara village in Pibor's Pochalla county, Abraham Kelang, Information Minister of Greater Pibor Administrative Area, said.
"Twelve people were killed, 10 injured and there were some cattle that were raided," Kelang said, adding that most of the dead were elderly.
The attackers, from the Murle ethnic group, are also suspected of abducting children, Owety Olung, the acting commissioner of Pochalla County, said.
"We have 15 children who are still missing right now. We don't know where they are if they are with the attackers or in the bush," Olung said.
Pochalla County is mostly inhabited by the Anyuak ethnic group. They and the Murle, who mainly live in neighboring Boma County, have fought sporadically.
The violence, partly motivated by cattle rustling, has involved ethnic groups from neighboring Jonglei State.
In late March, youths killed 15 people in Pibor, including Boma's deputy army commander, government officials and the county commissioner's bodyguards.
Officials also blamed the youths for the killing last year of another county commissioner in Pibor and a security official.
More than 150 people were killed earlier this year during conflicts between rival groups in northern and western South Sudan.