Aid workers in war-torn South Sudan have reached one of the worst hit areas where thousands are feared dying of starvation, the United Nations said but warned conditions may yet worsen.

Two teams of aid workers reached the town of Leer in southern Unity state, the UN said, scene of some of the heaviest fighting, including the mass abduction and rape of women and children.

"They brought with them life-saving supplies, including survival kits, high energy biscuits and blankets, to deliver to people in need who have been cut off from assistance since fighting resumed in October 2015," the UN Office for the Coordination of Humanitarian Affairs (OCHA) said in a report.

Civil war erupted in December 2013 when President Salva Kiir accused his former deputy Riek Machar of planning a coup, setting off a cycle of retaliatory killings that have split the poverty-stricken, landlocked country along ethnic lines.

Sources: MENA