The Commissioner of Mongalla County, Jubek, says two people have been killed in the latest attack on the area.
The unnamed deceased were a father and son.
The commissioner also said two other people who were wounded have been brought to Juba for further treatment.
Mr Elario Paul said some attackers came from an island called Juba na Bari. He said the situation is now calm.
“There is a group of criminals mobilized by a sultan called Abud Lado came by canoes and attacked an island next to Mongalla called Juba na Bari,” Mr Paul said, “and when people of this island responded, they run into the river.”
The motive of the attack is unclear.
Last month, however, gunmen attacked the area during the inauguration of the county commissioner. Two people were killed.
Officials from the neighboring Terekeka State later said the area was contested and the commissioner should not have conducted the ceremony without informing them.