JUBA, South Sudan - An angry mob of South Sudanese youths attacked a U.N. base on Thursday in order to harm members of an ethnic minority sheltering inside. Peacekeepers were forced to open fire, amid rampaging violence that killed at least 12 people, officials there said.

The U.N. mission in South Sudan condemned the "unprovoked attack" and called for a full investigation into the "heinous murders."

In a statement late Thursday, the U.S. ambassador to the United Nations, Samantha Power, said at least 20 civilians were killed and more than 70 injured in the "brazen, inhuman" attack.

The attackers forced their way into a camp in Bor in Jonglei state where some 5,000 ethnic Nuers have sought safety since fighting in South Sudan broke out in mid-December.

"The assailants, a mob of armed civilians, came to the base under the guise of peaceful demonstrators intending to present a petition ... The armed mob forced entry on to the site and opened fire on the internally displaced persons sheltering inside the base," said U.N. spokesman Stephane Dujarric, at U.N. headquarters in New York.

Maj. Major Kuol Mayen Deng of South Sudan's military said that Ugandan troops stationed in the region are now protecting the U.N. base. Deng said at least 12 people were killed. Two U.N. peacekeepers were injured in the attack, according to a U.N. statement.

"The violent attack happened despite repeated warning shots fired by U.N. peacekeepers. The armed mob forced entry onto the site and opened fire on the internally displaced person sheltering inside," the U.N. said.

Bor has changed hands four times between the military and rebels. The rebels back former Vice President Riek Machar, a Nuer who was fired from his office in July by President Salva Kiir, an ethnic Dinka.

U.N. Secretary-General Ban Ki-moon strongly condemned the attack on a site where civilians are being protected by the United Nations. Ban said that any attack on U.N. peacekeepers "constitutes a war crime," said U.N. spokesman Dujarric.

Ateny Wek, South Sudan's presidential spokesman, said the government was still receiving information about the attack. He called it a "very, very bad situation."

Ian Hughes, Britain's ambassador to South Sudan, said on Twitter that the attack was an "outrage."

"Internally Displaced People must be protected. Prospect for copycat attacks worrying," he wrote.

Fighting this week broke out in Unity state with anti-government forces taking control of Bentiu, while intense clashes were reported in Renk, north of Malakal in Upper Nile state.

Perry Mansfield, who heads aid group World Vision in South Sudan, said his group's Renk staff were caught in the crossfire and could not safely access the U.N. base.

"Renk was a stable place far from the conflict where parents could bring their children to safety while fighting continued. Now they are forced to run again," he said.

The violence has forced more than 1 million people in South Sudan to flee their homes. The planting season is under way but few are farming.

U.N. Secretary-General Ban Ki-moon said Wednesday that up to 1 million people face potential famine-level hunger because of the fighting.