Rebel fighters in South Sudan bombarded government positions Tuesday in the oil town of Bentiu, where 53,000 civilians are sheltering inside a United Nations camp, the defence minister said.

"The rebels are shelling our positions in Bentiu," Defence Minister Kuol Manyang told AFP. "This is a violation of the cessation of hostilities agreement, and we will act in self defence."

Aid workers in the northern town of Bentiu confirmed the shelling, saying they briefly took shelter in bunkers.

Fighting broke out in South Sudan, the world's youngest nation, in December 2013 when President Kiir accused his sacked deputy Riek Machar of attempting a coup.

The fighting in the capital Juba set off a cycle of retaliatory massacres across the country. Both government forces loyal to Kiir and rebels loyal to Machar continue to fight, despite numerous ceasefire deals.

Over half the country's 12 million people need aid, according to the UN, which is also sheltering some 100,000 civilians trapped inside UN camps ringed with barbed wire, too terrified to venture out for fear of being killed.

The latest fighting comes a day after donors pledged $529 million (467 million euros) towards a $1.8 billion aid appeal by the UN, with over 2.5 million people on the brink of famine.

UN aid chief Valerie Amos on Monday warned the warring leaders they had to show a committment to peace.

Kiir and rebel leader Riek Machar have been set a March 5 deadline to strike a final peace agreement, but previous deadlines have been repeatedly ignored despite the threat of sanctions.

The next round of faltering peace talks is due to resume on February 19.

Almost two million have been forced from their homes and 500,000 of them have fled abroad to neighbouring countries.