Juba: Fighting between South Sudanese rebels and government troops raged Monday, just days after a fresh ceasefire 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 ceasefire 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 Defence 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 told AFP.
Since President Salva Kiir and Machar signed a deal 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 -- who smear themselves in wood ash to ward off mosquitoes and as war-paint -- 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 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 organise elections in 2015, it would be good for South Sudan," Machar told reporters in the Ethiopian capital 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 Security Council, Ban cited a report by the UN mission in South Sudan which has said "there are reasonable grounds to believe crimes against humanity have been committed."
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 ceasefire 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.
On the government side -- backed by Ugandan troops -- the command structure under Kiir is seen as weak, while the rebels are made up of a loose coalition of army defectors and ethnic fighters.
Each side also accuses the other of using mercenaries and rebel forces from neighbouring Sudan.
The war in the world`s youngest nation has claimed thousands -- possibly tens of thousands -- of lives, with more than 1.2 million people forced to flee their homes.
UN rights chief Navi Pillay, a former head of the UN genocide court for Rwanda, has said she recognised "many of the precursors of genocide" listed in a report on atrocities released last week by the organisation.
The United Nations food agency has warned there is only a "small window of opportunity" to avert famine and appealed for relief agencies, who have been subjected to armed attacks and looting, to be allowed unfettered access.
The war erupted on December 15 with Kiir accusing Machar of attempting a coup. Machar then fled to the bush to launch a rebellion, insisting that the president had attempted to carry out a bloody purge of his rivals.