U.N. aid chief Valerie Amos arrived in war-torn South Sudan Monday as warnings mounted of a growing humanitarian crisis, with rebels and government accusing each other of breaking a ceasefire aimed at ending the conflict.

Amos, who arrived in Juba at the start of a three-day visit, is due to meet with government officials and aid workers in a bid to boost relief to the troubled young nation, where thousands have been killed and over 700,000 people have fled their homes in over six weeks of violence.

Sitting in a corner under a mosquito net in Bor hospital, two women recount a horrific ordeal at the hands of South Sudanese rebels.

Achin Mapio and Mary Yar, both patients, say they were repeatedly raped, had their food stolen and watched in terror as other patients were dragged out of the wards and shot.

In the shade of a tree close to the slow waters of the White Nile river, a body killed during the fighting over South Sudan's town of Bor gathers flies.

On the bumpy road towards the heavily fortified compound of the United Nations peacekeepers, another body of man lies beside the track.