An unknown number of people have been killed in two separate attacks in South Sudan.

At least 21 people were killed after gunmen ambushed vehicles on the Yei-Juba road on Sunday morning, a South Sudanese official said while blaming rebels for the attack.

The Eastern Nile State Information minister Jacob Lem Chan said four trucks were attacked and the victims were shot at and burnt.

"We have confirmed 21 dead, and about 20 casualties were admitted in the hospital," Mr Chan said, adding one truck was burnt with victims inside.

He said the dead were buried in a mass grave on Sunday, while some escaped to the bushes were they could still be.

"These are SPLA-IO force ... who laid the ambush," Mr Chan said, referring to former vice president Riek Machar's rebel movement.

Juba-Nimule attack

Several people were feared dead and others abducted after three buses were ambushed along the Juba-Nimule road on Monday morning.

While dismissing reports of death, Uganda's army spokesman Col. Paddy Ankunda said of the three vehicles, Echo bus -owned by a South Sudanese, was burnt and its driver injured.

The other two Ugandan-owned buses --Gateway and Friendship-- were escorted to Uganda by the South Sudan army after they rescued the passengers, Col. Ankunda said.

Since renewed fighting broke out in July between forces loyal to President Salva Kiir and those of Dr Machar, there has been increased road attacks that has forced the South Sudan government to provide security along the Juba-Nimule road for transportation of food and traders from Uganda.