Some 17 passengers were killed plus four others in separate incidents, Eastern Equatoria Information Minister says.
Mark Akio says government forces are pursuing the unidentified bus and car attackers at Amee junction where the incident happened.
The state minister wonders why there are still attacks after government has announced commitment to peace implementation.
Minister Akio says there is no suspect linked to the highway ambush.
He adds that government is in control of the situation.
Movement on Nimule-Juba Highway was blocked on Thursday morning following the attack, sources say.