More than 300,000 people are without "life-saving" aid in South Sudan's oil-rich Unity state after heavy fighting forced aid agencies to withdraw, the BBC quoted an UN report as saying on Tuesday.

Government forces have been advancing towards Leer, the birthplace of rebel leader Riek Machar, reports say.

Emergency relief has come to a stop in areas worst-affected by fighting, the UN said.

International mediation efforts to end the 17-month conflict have failed.

South Sudan is the world's newest state, gaining independence from Sudan in 2011.

Violence broke out in December 2013 when President Salva Kiir accused Machar, his sacked deputy, of plotting a coup.

Machar denied the allegation, but then formed a rebel army to fight government troops.

"Ongoing hostilities in Unity state have now obliged all non-governmental organisations and UN agencies to evacuate staff from Leer and other locations," said UN aid chief for South Sudan Toby Lanzer.

"As a consequence, over 300,000 civilians who are in need of emergency relief, including food aid and medical services, do not currently have access to such life-saving assistance."

On Friday, the UN said that up to 100,000 people had fled fighting in Unity state in the first week of this month.

"Renewed violence in southern Unity comes at a time when stocks of food are depleted, and precisely at the height of the traditional planting season when civilians could be planting their crops in order to reap a harvest later this year," Lanzer said in a statement.