The Speaker of Western Equatoria State Assembly, James Bage Elisa, was shot dead by armed gunmen in a road ambush in Yambio on Friday night.
The newly appointed Government of Western Equatoria State, Major Patrick Zamoi, has condemned the killing.
According to Eye radio, they called him for a meeting there at night.
While returning home with his bodyguard, Elisa fell in an ambush at Uze bridge a few kilometres from Yambio at around 8 pm.
The gunmen shot him dead but the bodyguard escaped.
Authorities in the state say Elisa will be buried in the assembly premises.
Elisa’s killing comes after the murder of 10 civilians were killed during a Thursday morning attack by the Sudan People’s Liberation Army on the Leer area of Unity state.
Rial Gatdet Weil, the information secretary of the rebel-appointed state government told Radio Tamazuj the government forces appeared to be targeting civilians and that no SPLA-IO forces were even in the area where the attack occurred.
At the same time, a high-ranking SPLA-IO officer who requested anonymity, accused the government of carrying out aerial bombardments in the Gabaat area of northern Upper Nile state.
SPLA spokesperson Colonel Philip Aguer confirmed fighting between the government and SPLA-IO in Gabaat, but did not have further details about the battle.
Another politician missing
Meanwhile, family members of renowned South Sudanese politician, Peter Abdulrahman Sule, told Radio Tamazuj that he has been missing in Uganda since last month and has not resurfaced.
Peter is a lawyer, politician and the leader of the United Democratic Front party in South Sudan.
He has gone missing since 24 July 2015, according to relatives.
They say that he had gone to Adjumani in northern Uganda to visit some South Sudanese friends but was abducted by plainclothes security personnel.
Nobody knows whether those who arrested him were Ugandan or South Sudanese security operatives or somebody else.
Last year the South Sudanese government prevented Sule from traveling to peace talks in Addis Ababa.
He sought asylum in Uganda.
Family say that he is a fully recognised refugee in Uganda under the Geneva Convention of 1951.