Warring forces in South Sudan battled Tuesday in the oil-rich Upper Nile state, the defence minster said, claiming insurgents were trying to capture a key town ahead of peace talks.

Heavy shelling was reported in several sites in the northern Renk region on both Monday and Tuesday, Defence Minister Kuol Manyang told AFP, resulting in the "injuring of many civilians... and destroying a number of houses."

There was no information on how many might have been killed.

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.

Over two dozen armed forces -- including government soldiers and allied militia backed by Ugandan soldiers on one side, and a range of rebel factions on the other -- have been battling it out for the last 14-months despite numerous ceasefire agreements.

Regional bloc IGAD has set a March 5 deadline for Kiir and Machar to reach 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.

Manyang said rebels were making a grab for more territory to increase their bargaining power at the talks.

"The rebel leader (Machar) says he will only sign a peace agreement when he is in control of a strategic town," said Manyang, claiming that rebels were also "mobilising now to attack Bentiu," the capital of Unity state which was shelled earlier this month.

Fighting began in the capital Juba setting off a cycle of retaliatory massacres across the country.

The UN estimates that 2.5 million people are in a state of emergency or crisis, just steps short of famine.

No overall death toll for the war has been kept by the government, rebels or the UN, but the International Crisis Group estimates that at least 50,000 people have been killed.