A mob of armed civilians pretending to be peaceful protesters delivering a petition to the United Nations in South Sudan forced their way into a UN base sheltering some 5,000 civilians and opened fire, the world body said.
A UN source, speaking on condition of anonymity, said at least 20 people had been killed and 60 wounded in the attack on Thursday on the base in Bor in northern Jonglei state, where there are Indian and South Korean UN peacekeepers.
The source warned that the death toll was likely to rise.
UN spokesman Stephane Dujarric said dozens of civilians were wounded, but the exact number of people killed or wounded had not yet been confirmed.
Two UN peacekeepers were wounded repelling the armed mob, he said.
More than 1 million people have fled their homes since fighting erupted in the world's youngest country in December between troops backing President Salva Kiir and soldiers loyal to his sacked vice President, Riek Machar.
Thousands of people have been killed and tens of thousands have sought refuge at UN bases around the country.
"This attack on a location where civilians are being protected by the United Nations is a serious escalation," Dujarric said.
"The assailants, a mob of armed civilians, came to the base under the guise of peaceful demonstrators intending to present a petition to UNMISS (the UN peacekeeping mission)."
"The armed mob forced entry on to the site and opened fire on the internally displaced persons sheltering inside the base," he said. "At the time of the attack there were some 5,000 displaced civilians ... inside the base."
Dujarric said the wounded were being treated at the UN compound.
On January 23, the warring parties agreed to a cessation of hostilities but fighting has continued in parts of South Sudan, which seceded from Sudan in 2011 under an agreement to end decades of war.
The current conflict has disrupted oil production, which provides a hefty portion of the government's revenue. South Sudanese rebels said on Tuesday they have seized the capital of oil-producing Unity state, Bentiu, and warned oil firms to pack up and leave within a week.