KHARTOUM -- The United Nations said on Monday that more than 300,000 people need emergency relief in South Sudan's oil-rich Unity State.

"Ongoing hostilities in Unity State have now obliged all non-governmental organisations and UN agencies to evacuate staff from Leer and other locations," said Toby Lanzer, the UN Humanitarian Coordinator in South Sudan, in a statement on Monday.

He said that the humanitarian response south of Bentiu in Unity State has "come to a stop."

"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," Lanzer said.

According to the United Nations, around 100,000 people have been uprooted last week due to renewed clashes in South Sudan.

Due to heavy fighting, the Medicines Sans Frontiers and the International Committee of the Red Cross said separately on Saturday that they have pulled out their staff from Leer.

South Sudan, which became independent in 2011, plunged into violence in December 2013, when fighting erupted between troops loyal to President Salva Kiir and defectors headed by his former deputy Riek Machar.

The conflict soon turned into an all-out war, with violence taking on an ethnic dimension that pitted the president's Dinka tribe against Machar's Nuer ethnic group.

The clashes have left thousands of South Sudanese dead and forced around 1.9 million people to flee their homes.