A mob of armed civilians pretending to be peaceful protesters delivering a petition to the UN in South Sudan forced their way into a UN base sheltering some 5,000 civilians and opened fire, killing dozens.
A UN source said at least 58 people had been killed and dozens wounded in the attack on the base in Bor in northern Jonglei state on Thursday, where there are Indian and South Korean UN peacekeepers.
Forty-eight bodies, including children, women, men, have been recovered from inside the base. The bodies of 10 attackers have been found outside the base.
The total death toll could increase as over 100 people were wounded, some of them very seriously.
More than one 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.
Dujarric said the wounded were being treated at the UN compound.
US Ambassador to the United Nations, Samantha Power, condemned all recent attacks on civilians and the United Nations in South Sudan, saying "particularly egregious is today's armed attack on an UNMISS compound in Bor by a heavily armed group that used rocket-propelled grenades."
"All parties should regard UNMISS sites as inviolable, and should afford protection to citizens sheltering at those locations," Power said in a statement.
"The US will work with our international partners to establish who was responsible for, or complicit in, this horrific attack."