Dozens of civilians sheltering in a UN base in the South Sudan town of Bor have been killed in an attack by armed youths, the UN says.
Toby Lanzer, the UN's top aid official in South Sudan, told the BBC that the youths broke through the gates and opened fire.
UN peacekeepers returned fire and eventually repelled the attackers, he said.
Almost 5,000 civilians are sheltering at the base in the war-ravaged town.
Thousands of people have been killed in South Sudan since fighting began in December between supporters of President Salva Kiir and his former deputy, Riek Machar.
More than one million people have fled their homes in the conflict, some to neighbouring countries.
Mr Lanzer said a group of about 350 youths had left the centre of Bor, in Jonglei state, and approached the base on Thursday morning, allegedly wanting to present a petition.
"They managed to force the gate open, they came in and started shooting indiscriminately," he told the BBC.
"It is the bravery of the peacekeepers that managed to repel the attack. Unfortunately we have had significant loss of life. I can't confirm the number but I can tell you it runs into the dozens."
Mr Lanzer, the UN assistant secretary general in South Sudan, said the youths were dressed in civilian clothing and it was not clear who they were.
"It was totally unprovoked and I think that meting out violence on a group of civilians who are sheltering and seeking protection from the United Nations is not only cowardly it is abominable," he said.
Mr Lanzer added that the UN had stepped up security following the attack.
"We have further reinforced our base and we will send very clear signals to anybody who wishes to approach us with any intent of wrongdoing," he said.
"We've got clear rules of engagement and we will use force if at all necessary to protect people whose sole purpose for being inside our base is to stay alive."
South Sudan seceded from Sudan in 2011 after a long and bloody conflict to become the world's newest state.