A Chinese national and South Sudanese soldier were killed on Wednesday along a major road linking the capital Juba to the town of Rumbek, a regional government official said.
The almost 400-kilometre road is undergoing major reconstruction by the Shandong Hi-Speed Group Company at a cost of some $700 million dollars.
"A Chinese (citizen)... and a soldier belonging to the Tiger battalion have been killed," along the road, said the information minister for the Central Equatoria state, Wayi Godwill.
The pair were "attacked, shot and died instantly. The motive of the killing is unknown and until now the assailants are at large", he added.
The murders come a week after South Sudan security officers arrested 12 suspects accused of carrying out a spate of abductions, kidnappings for ransom, torture and killings of civilians -- mainly businessmen or women -- along major roads in the country.
Insecurity in the country persists even after the signing of a 2018 peace deal to end a brutal five-year civil war that has left some 400,000 dead.
Numerous attacks on civilians and commercial vehicles have taken place in recent months on the Juba-Yei road, as well as the Juba-Nimule road leading to Uganda.
The Kenyan government raised the alarm over the situation in April after saying an attack on Kenyan truck drivers on the Juba-Nimule road had resulted in "several fatalities and multiple injuries".
Independent radio station Eye Radio reported several Ugandans had also been killed in attacks. Last month scores of trucks refused to enter the country until they had a security escort.