Unknown gunmen on Tuesday ambushed a commercial vehicle on the main road connecting South Sudan's capital with the Ugandan border, killing at least two people, police authorities said.

"The Toyota Noah was attacked in Aswa village when it was coming from Juba to Nimule without an escort," Daniel Justin, South Sudan police spokesperson told Radio Tamazuj on Wednesday.

He said the vehicle was targeted by gunmen, pointing out that an investigation was underway.

Tuesday's ambush was the first this year on the Juba-Nimule road.

The highway is a lifeline for the youngest nation, which imports almost everything, and for people traveling to neighboring Uganda.