At least 58 people were killed Thursday when a group 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 and opened fire, the world body said.

Among those killed in the attack on the base in Bor in northern Jonglei state were 46 internally displaced persons and 12 attackers, according to the U.N. Mission in South Sudan. No U.N. staff were killed, they said.

Another U.N. spokesperson said about 100 had been injured. Two U.N. peacekeepers were wounded repelling the armed intruders, the U.N. said.

The United Nations has reinforced security at Bor, and South Sudan's government has sent troops to secure the base.

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.

Dujarric said the wounded were being treated at the U.N. compound.

On Friday, a surgical team from Doctors Without Borders flew into Bor to help treat those injured in the attack, the U.N. said.

The U.S. State Department condemned the attack.

"We reiterate our call upon the Government of South Sudan to end the violence and to fulfill its primary responsibility to maintain law and order and provide full support for the UNMISS mission to protect civilians," State Department spokeswoman Marie Harf said in a press release.

Her statement also condemned recent attacks and counter-attacks in the town of Bentiu by anti-government and pro-government forces in violation of a Jan. 23 Cessation of Hostilities agreement.

"The deliberate targeting of civilians during these attacks is unacceptable and those responsible for such acts must be held accountable," Harf said.

The statement called on both sides in the conflict to honor the Jan. 23 agreement and enter into an inclusive political dialogue.

South Sudanese rebels said Tuesday that they have seized Bentiu, the capital of oil-producing Unity state, and warned oil firms to pack up and leave within a week.

South Sudan 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.

Al Jazeera and Reuters