South Sudan President Salva Kiir has postponed the country's 2015 presidential election amid renewed fighting between the army and rebel forces, news agency reports say.

Kiir said the country's warring factions would need time to reconcile, with a new poll possibly taking place as late as 2018.

Kiir and rebel leader Riek Machar, who was sacked as Kiir's deputy in July 2013, signed a new ceasefire agreement in Ethiopia on Friday and said they would hold further talks about forming an interim government to end the nearly five months of bloodshed.

Despite the ceasefire, fresh battles continued to rage on Monday with both sides accusing each other of attacking the other for a second day.

Defence Minister Kuol Manyang told the AFP news agency that fighting raged in the oil-producing state of Upper Nile, adding that government troops had been ordered "not to go and attack, but only to fight in self defence."

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.

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

Source: Agencies