By Hereward Holland
JUBA (Reuters) - More than 100 people have been killed in South Sudan in an attack by rebels and ethnic allies on a convoy of families from a rival tribe and their cattle, an official said on Sunday.
Since breaking from Sudan in 2011, oil-producing South Sudan has struggled to assert control over remote territories awash with weapons after a 1983-2005 war with the north and torn by ethnic rivalries.
The attack on Friday was the worst violence in Jonglei State since 900 people were killed there in tribal attacks linked to cattle rustling in 2011, the United Nations said.
Rebels loyal to former theology student David Yau Yau and members of the Murle community had killed 103 people, most of them women and children, in the ambush on ethnic Lou Nuer families, state governor Kuol Manyang said.
"They came under attack from people in a huge force," he told Reuters. "There are many children and women missing. Their fate is not yet known."
Fourteen soldiers escorting the convoy were also killed, he said.
The International Committee of the Red Cross said it had sent a medical team to treat the wounded.
Yau Yau rebelled in July last year. He recruited armed youths antagonized by a government campaign to end tribal violence in Jonglei, which human rights groups say was marked by abuses by soldiers.
More than 1,500 people have been killed in Jonglei since independence, according to the United Nations.
South Sudan accuses Sudan of dropping weapons and ammunition to Yau Yau's rebels, an allegation denied by Khartoum.
The violence in Jonglei is hindering government plans to explore a major oil concession with the help of France's Total.
(Editing by Ulf Laessing and Matthew Tostevin)