Rebels attacked trucks carrying civilians in South Sudan, killing 21 people, the government said, as violence between rival forces loyal to President Salva Kiir and his former vice president risks dragging the country back into civil war.The United Nations Mission in South Sudan (UNMISS) said in a statement on Monday it had received reports of horrific violence being perpetrated against civilians in Central Equatorial State and urged both rebel and government commanders to control their forces and cease hostilities.The government said on Monday 21 civilians were killed, some burned to death, and about 20 wounded when rebel gunmen ambushed their vehicles on a road connecting Yei town, in Central Equatorial State, to the capital Juba on Saturday.Jacob Lem Chan, a local government official, said four trucks were ambushed and victims were burned and shot at."These are SPLA-IO force... who laid the ambush," Chan said, referring to former vice president Riek Machar's rebel movement."We have confirmed 21 dead, and about 20 casualties were admitted in the hospital," Chan said, adding one truck was burnt with victims inside.Dickson Gatluak Jock, military spokesman for SPLM-IO, denied its forces had carried out the attack and said it directed attacks against military installations only. "We are not intending to harm civilians or to kill them," he said.Violence has been growing in Africa's youngest nation since July when the rival forces battled each other in Juba.Machar fled the capital to Sudan from where he has urged his forces to re-organize for "armed resistance" to Kiir's government.Rivalry between Kiir, an ethnic Dinka, and Machar, a Nuer, led to South Sudan's first war in December 2013 when Kiir sacked Machar as his vice president.The conflict ended in a peace pact in August 2015 and Machar returned to Juba and was sworn in as vice president in April.But there has been frequent violations of the peace agreement and major fighting broke out again in July. Machar has since been replaced as vice president by General Taban Deng Gai.