At least 58 people were killed and more than 100 others wounded in Thursday’s attack on a UN base in South Sudan sheltering thousands of displaced civilians, a UN official said.

At least 58 people were killed and more than 100 others wounded in Thursday's attack on a UN base in South Sudan sheltering thousands of displaced civilians, a UN official said.

"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 top UN official in the country, Toby Lanzer, told AFP on Friday.

"The total death toll is 58, but that could increase as over 100 people were wounded, some of them very seriously."

In the clearest account yet of the incident in the government-controlled town of Bor, Lanzer said a group of around 350 armed youths in civilian clothes "used extremely violent force to breach the perimeter" of the UN base.

He said they opened fire on terrified civilians, who have sought shelter with the UN from a wave of ethnic violence that has marked the four-month-old conflict, 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.

He praised the actions of UN peacekeepers from India, Nepal and South Korea charged with the protection of the 5,000 people in the UN base.

"We will do everything necessary to protect the lives of people in our protection, including the use of lethal force," Lanzer said.

He said measures had been taken to boost security at other UN bases in the country, which are sheltering close to 60,000 people from different ethnic groups.

"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.

Lanzer 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 fallen into "a cycle of revenge".

"It's vital that all communities realise that they are taking this country nowhere fast," he said.