A reported 48 people were killed and 60 people wounded Thursday when armed civilians, posing as peaceful protesters, attacked a United Nations base in South Sudan that was sheltering close to 5,000 civilians.

Two Indian peacekeepers also were severely wounded in the attack on the base in Bor, in northern Jonglei state, according to news reports. The U.N. base was giving shelter to civilians who had fled their homes since violence erupted between troops backing President Salva Kiir and soldiers loyal to his sacked vice president, Riek Machar. The wounded were reportedly being treated in the U.N. compounds.

"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," said U.N. spokesman Stephane Dujarric, according to Reuters. "The armed mob forced entry on to the site and opened fire on the internally displaced persons sheltering inside the base."

Nancy Powell, a U.S. envoy to the U.N., said the latest attack is "particularly egregious” and condemned such attacks on civilians by an “armed group that used rocket-propelled grenades."

"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, according to Reuters.

More than a million South Sudanese have been dislocated from their homes since December even as the conflict has disrupted oil production, the country's main source of revenue. South Sudanese rebels said Tuesday they have seized the capital of Bentiu state, which produces oil, and warned oil companies in the region to leave within a week.

“UNMISS calls on all displaced persons sheltering in its bases to remain calm, exercise restraint, strictly respect the ground rules governing their stay in the camps and desist from any actions that may provoke violence,” the U.N. mission said in a statement Friday.