About 30 people have been killed in two separate attacks on passenger vehicles in South Sudan, the army and police said Monday.
In the first incident, gunmen attacked a passenger lorry near Yei in the south-west on Saturday, army spokesman Lul Ruai Koang said.
They killed 21 people, wounded another 20 and set the lorry on fire, he added. The fatalities included 15 children.
The UN mission to South Sudan said it had received "deeply disturbing reports of horrific violence perpetrated against innocent and vulnerable civilians” in the area.
In another incident near the capital Juba on Monday, gunmen ambushed three buses travelling to Uganda and killed nine people, police spokesman Justin Boula said.
Six other passengers had bullet wounds. Fifteen went missing, but they may have gone into hiding after fighting broke out between the attackers and the army, Boula added.
Ruai Koang and Boula blamed the incidents on rebels fighting President Salva Kiir.
South Sudan sank into chaos when a power struggle between Kiir and his former deputy Riek Machar escalated into a military conflict in December 2013.
Tens of thousands have been killed and about 2.5 million displaced.
The formation of a unity government in April did not stop the fighting, and Machar later fled the country.