"Any attack on United Nations Peacekeepers is unacceptable and constitutes a war crime," Ban said in a statement released hours after the attack.
Ban also called the attack a "serious escalation" of the four-month-old crisis in South Sudan.
#SouthSudan: #UNSG ‘strongly condemns’ attack on @unmisspio in Jonglei state http://t.co/RN0MzgcBy3 @UNPeacekeeping pic.twitter.com/1GDwmsy2Tz
— UN News Centre (@UN_News_Centre) April 17, 2014
The U.N. Mission in South Sudan (UNMISS) said in a statement that an armed mob forced entry to the U.N. compound in Bor, the capital of Jonglei state, and "opened fire on the internally displaced persons sheltering inside the base."
UNMISS said two U.N. peacekeepers were wounded in he attack, and an unconfirmed number of civilians were killed or wounded by the "assailants who came under the guise of peaceful demonstrators intending to present a petition to UNMISS."
The U.N. will make public the exact casualty figures once it has determined what they are, the statement said.
UNMISS strongly condemns today’s deadly & unprovoked attack on its base in Bor, the capital of Jonglei State, by a mob of armed civilians.+
— UNMISS (@unmisspio) April 17, 2014
Bior Kuer, a health worker at Bor Hospital, said at least two people were killed in the fighting at the compound and 14 others were treated at the hospital in town for wounds sustained during the violence. UNMISS said others were treated at the medical clinic inside the compound.
William Oyual, a health worker who lives inside the UNMISS compound, said scores of internally displaced persons (IDPs) sheltering at the U.N. facility were killed in the fighting.
“Those who are dead are 61 and the wounded are 273," said Oyual, who has been helping to treat the wounded.
Oyual said the large group that stormed the base were reportedly angered at hearing that displaced people inside the compound were celebrating the news that Bentiu, in Unity state, was recaptured by rebels at the start of the week.
The assailants "started shooting UNMISS" when the U.N. peacekeepers refused them entry to the compound, Oyual said.
"UNMISS has no mandate to shoot them, so UNMISS ran down to their protection area" and the attackers followed, shooting indiscriminately at people, Oyual said.
Women and children were among the victims, he said.
In its statement, UNMISS confirmed that the attackers "opened fire on the internally displaced persons sheltering inside the base."
Ayuen Deng Ror, one of the organizers of the protest outside the UNMISS compound, said the first shots were fired by people inside the camp, although he could not tell if U.N. peacekeepers were responsible or if displaced people opened fire.
UNMISS said peacekeepers tried unsuccessfully to stop the attack by firing warning shots and also "returned fire" after the assailants began firing on civilians in the camp.
The U.N. has a strict policy banning civilians from carrying weapons into its compounds or bases. In January, a South Sudanese government minister was barred entry to the UNMISS compound in Bor because his bodyguards were armed.
Ror said the demonstrators wanted UNMISS to expel the people who were celebrating the recapture of Bentiu by forces loyal to former vice president and opposition leader Riek Machar.
Sudan People’s Liberation Army (SPLA) director of information Brigadier General Malak Ayuen condemned the "senseless violence" and deaths of South Sudanese during an interview broadcast on national television. The military spokesman called on "members of the civil population and UNMISS... not to take the law into their hands."
UN frustrated by 'pointless violence'
U.N. humanitarian coordinator for South Sudan Toby Lanzer said in a statement that he was "deeply saddened and frustrated by the violence that has ravaged Bentiu and Bor in the past 72 hours."
Press Release: "...no excuse for direct attacks on civilians or on those risking their own lives to protect them." http://t.co/SDHlTREYiL
— Toby Lanzer (@tobylanzer) April 17, 2014
"These events show, yet again, the pointlessness of the violence engulfing South Sudan," he said. "The current cycle of revenge will get the people of this country nowhere. It wrecks the present and casts a dark shadow over what should have been a very bright future."
South Sudanese army spokesman Colonel Philip Aguer called the clashes in Bor “unfortunate” and said the military will launch an investigation to find out what happened.
The fresh outbreaks of violence in Bentiu and Bor come nearly three months after the government and opposition signed a cessation of hostilities agreement at peace talks in Addis Ababa. Violence has also been reported in Upper Nile state.
The United States has warned that it will slap sanctions and travel bans on anyone who blocks South Sudan's slow-moving peace process or io. Humanitarian leaders from the European Union, the United Nations and United States have warned that South Sudan could face famine if the fighting does not stop soon.