The Carter Center said in a statement on the vote tabulation phase that the process was marred by major incidents in four states in South Sudan.
The statement release yesterday singles out problems in Western and Central Equatoria, Unity states and Northern Bahr el Ghazal.
The document was prepared with reports from Carter Center observers at State High Committee tabulation centres across the country.
The document says in Western Equatoria ballot votes were burnt in two places.
It added that SPLA troops dressed as police interfered with the final counting in Yambio, arresting the head of the data centre and one of his assistants.
In Central Equatoria the statement questioned the turnout in four state constituencies in Terekeka County.
It added that unidentified security personnel removed from the State High Committee computers and the results of the governor’s race from 14 constituencies without justification.
The statement said the elections in Unity State suffered from large-scale intimidation, violence, flaws in administration, and indications of manipulation, especially in Leer and Pariang counties.
Finally, in Northern Bahr al Ghazal, the Carter statement said that the incumbent governor and county commissioners committed serious irregularities during polling and counting.