(Updates death toll in paragraph two, adds comment by U.S. envoy to United Nations)
By Michelle Nichols
UNITED NATIONS, April 17 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 U.N. base sheltering some 5,000 civilians on Thursday and opened fire, the world body said.
A U.N. source, speaking on condition of anonymity, said at least 48 people had been killed and 60 wounded in the attack on the base in Bor in northern Jonglei state, where there are Indian and South Korean U.N. peacekeepers.
U.N. spokesman Stephane Dujarric said dozens of civilians were wounded, but the exact number of people killed or wounded had not yet been confirmed. Two U.N. 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 U.N. 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 U.N. 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 U.N. compound.
U.S. 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 United States will work with our international partners to establish who was responsible for - or complicit in - this horrific attack and seek to bring the perpetrators to justice," she said.
On Jan. 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 U.S. State Department called on both sides in the conflict to honor the Jan. 23 agreement and enter into an inclusive political dialogue.
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. (Reporting and writing by Michelle Nichols; Additional reporting by David Brunnstrom in Washington; Editing by Leslie Adler, Andrew Hay and Cynthia Osterman)