Fighting between South Sudanese rebels and government troops raged on Monday, just days after a fresh cease-fire deal, dashing hopes to a swift end to five months of civil war.

Both sides accused each other of attacking the other for a second day since the cease-fire officially came into force, with each claiming they are defending their positions.

Rebel chief Riek Machar said government forces had been on "a continuous offensive", while Defense Minister Kuol Manyang reported insurgent attacks in the oil-producing state of Upper Nile.

Government troops had been ordered "not to go and attack, but only to fight in self defence", Manyang said.

Since President Salva Kiir and Machar signed a deal on Friday to halt fighting, both sides have blamed each other for launching ground attacks and artillery barrages.

Machar was "not in control of his forces", Manyang said. He claimed heavily armed militia known as the White Army had attacked government troops.

"These are irregular forces ... they do not know about the cessation of hostilities agreement that was signed," he added.

Rebel spokesman Lul Ruai Koang said the army on Monday wrested back control of the flashpoint town of Bentiu - which the government said it had already recaptured last week - and charged the army with "indiscriminate, intensive and extensive shelling of surrounding villages".

Kiir said on Sunday that elections due in 2015 would be postponed for "two or three years" to allow "reconciliation among the people", prompting a furious response from Machar, a sacked vice-president who has said he wants to compete for the top job.

"If he (Kiir) were sincerely committed to peace, he would organize elections in 2015, it would be good for South Sudan," Machar told reporters in Addis Ababa, Ethiopia, on Monday.

UN Secretary General Ban Ki-moon raised the possibility of a "special tribunal" to try those responsible for abuses in South Sudan.

In remarks to the UN Security Council, Ban cited a report by the UN mission in South Sudan that said "there are reasonable grounds to believe crimes against humanity have been committed".

"My message to both leaders was clear: They must work together to heal the wounds they have opened," Ban said.

"If the conflict continues, half of South Sudan's 12 million people will either be displaced internally, refugees abroad, starving or dead by the year's end," Ban warned the Security Council.

The conflict, which started as a personal rivalry between Kiir and Machar, has seen the army and communities divide along ethnic lines, pitting members of Kiir's Dinka tribe against Machar's Nuer.

Machar said the violence had created a "cycle of revenge", but that he still wanted an end to the war.

"I have no personal anger," Machar said about his rival. "I hope the peace process will succeed."

The two sides had agreed to a cease-fire in January, but that deal quickly fell apart and unleashed a new round of fierce fighting.

Observers have said both sides will face challenges in implementing a truce.

The war in the world's youngest nation has claimed thousands of lives, with more than 1.2 million people forced to flee their homes.

AFP-AP-Reuters