The top UN official in the war-torn nation, Toby Lanzer, praised peacekeepers from India, Nepal and South Korea for preventing what could have been a massacre of up to 5,000 people, and vowed the world body would use "lethal force" again to protect civilians under their protection.
"We will do everything necessary to protect the lives of people in our protection, including the use of lethal force," Lanzer told AFP.
He said they opened fire on terrified civilians, who have sought shelter with the UN from a wave of ethnic violence, with the apparent aim of killing as many people as possible.
"When we realised we were under attack we responded... The quick actions of the peacekeepers saved lives," Lanzer said.
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"Forty-eight bodies, including children, women, men, have been recovered from inside the base. The bodies of 10 attackers have been found outside the base. The total death toll is 58, but that could increase as over 100 people were wounded, some of them very seriously," he said.
"This past week has been the most bleak in South Sudan's history," Lanzer said, citing the attack on the UN base as well as reports of renewed atrocities further north in the oil-hub of Bentiu, which fell to rebel forces during the week.
He said South Sudan's conflict, which began on December 15 following a clash between army units loyal to President Salva Kiir and troops backing ousted vice president Riek Machar, had now descended into "a cycle of revenge".
The UN's aid agency UNOCHA said it was "particularly outraged by deliberate and targeted killings of civilians in hospitals, churches, UN peacekeeping bases and other places where people's rights should be sacrosanct".
Most of the civilians hiding in the UN base in Bor are thought to be ethnic Nuer, the same tribe of Machar, who now leads a rebel force comprised of ethnic militia and army defectors. During the week the rebels captured the town of Bentiu, a key oil hub in the north.