In the most recent attack on the same road in June at least 15 people were killed and nine wounded
At least four people were killed by unknown gunmen on the main road between South Sudan's capital and the Uganda border on Wednesday, an army spokesman said.
The deadly attack was the latest on the dangerous yet busy highway that ferries South Sudanese fleeing to Uganda or those returning home to Juba. The identities of the victims had not been established by press time.
"Four civilians died on the spot and five others were wounded," said army spokesman Brigadier General Lul Ruai Koang.
Koang said the attack on the Juba-Nimule road involved machine gun fire and rocket-propelled grenades. The death toll is expected to rise.
Two of the attackers were also killed when soldiers escorting the convoy returned fire.
In the most recent attack on the same road in June at least 15 people were killed and nine wounded.
That ambush was claimed by the rebel SPLA-IO group, led by former deputy president Riek Machar, saying it was in retaliation for attacks on its positions by governments forces loyal to President Salva Kiir.
Rebel spokesman Lam Paul Gabriel denied responsibility for Wednesday's ambush.
Despite the risks the highway remains a lifeline for South Sudan, which imports almost everything, as well as for people fleeing the more than three-year-long civil war.
Since the government began deploying military escorts for convoys using the road in January there have been at least five separate deadly attacks