At least 3.9 million people in South Sudan are facing severe food insecurity, three United Nations agencies have warned.

"Extreme hunger is pushing people to the brink of a catastrophe in parts of South Sudan as a new analysis found that 3.9 million people nationwide face severe food insecurity," Xinhua cited from a joint statement by the Food and Agriculture Organisation (FAO), the United Nations Children's Fund (UNICEF) and the World Food Programme (WFP) on Thursday.

"A newly released Integrated Food Security Phase Classification analysis showed that at least 30,000 people were living in extreme conditions and were facing starvation and death," said WFP Country Director Joyce Luma in the statement.

Luma said people in South Sudan need peace, nutritious food and other humanitarian assistance and livelihood support to survive and rebuild their lives.

For his part, UNICEF Representative in South Sudan Jonathan Veitch said that "since fighting broke out nearly two years ago, children have been plagued by conflict, disease, fear and hunger".

According to the statement, unless unrestricted humanitarian access was urgently granted, food insecurity could deteriorate to famine in parts of Unity State, where humanitarian assistance had been hampered by the dreadful violence and lack of access in recent months.

South Sudan was plunged into violence in December 2013 when fighting erupted between troops loyal to President Salva Kiir and defectors led by his former deputy Riek Machar.

The conflict soon turned into an all-out war with the violence taking on an ethnic dimension.

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