Rebels in South Sudan said they were closing in on key oil fields and two state capitals, predicting an imminent collapse of the government.

A statement from the rebels' spokesman, General Lul Ruai Koang, said on Thursday that forces battling President Salva Kiir captured the town of Renk, close to the border with Sudan, on Wednesday and were advancing on the Paloich oil fields, according to the AFP news agency.

"The fall of Renk ... leaves government troops trapped in Malakal with no supply and escape routes," he said, referring to the strategic capital of Upper Nile state which has already changed hands several times in the four-month-old conflict.

He also said the rebels "once again renew calls for oil companies to stop production and evacuate staff/employees to avoid being caught in crossfire".

The spokesman also said rebels loyal to former Vice President Riek Machar were advancing on Jonglei State's capital Bor, situated just 200km north of the capital Juba.

The rebel claims could not be independently confirmed.

"While our forces are making steady military gains ... Kiir's military leadership is unravelling," Koang said, the day after the president sacked his army chief and head of intelligence in the wake of the loss of Bentiu, another northern state capital and oil hub that fell to the rebels last week.

He also said the purge "marks the beginning of an imminent bloodbath, escalation and regionalisation of the conflict" in the world's youngest nation, which only won independence from Khartoum in 2011, after decades of violence and war.

Herve Ladsous, UN peacekeeping chief, demanded an immediate end to the violence and warned that with the changing season, a "humanitarian catastrophe" would become more certain.

He said a million people were at risk of famine if the war didn't stop.

More than one 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 deputy, Riek Machar.

The UN accused the rebels of hunting down men, women and children last week in a hospital, church and mosque in the capital of the oil-producing Unity state and then killing them based on ethnicity and nationality, saying the attack constituted a war crime.

After the rebels seized Bentiu, Dinka residents of Bor town in Jonglei state attacked a UN base on Thursday where about 5,000 people, mostly Nuer, were sheltering, according to Reuters.

They pretended to be peaceful protesters delivering a petition to the UN before opening fire on the base, killing some 58 people and wounding 98, including two Indian peacekeepers, the UN said.

Security Council members are considering sanctions on South Sudan's warring parties, envoys said, demanded "serious consequences" be imposed to force an end to the violence.

Thousands of people have been killed and tens of thousands have sought refuge at UN bases around South Sudan after the violence spread across the country.