South Sudan's government is mainly to blame for famine in parts of the war-torn country, yet President Salva Kiir is still boosting his forces using millions of dollars from oil sales, a confidential United Nations report alleges.

Key points: South Sudan's government made $316 million from oil sales in about seven months last year, the UN says

South Sudan's government made $316 million from oil sales in about seven months last year, the UN says At least half of the country's budget is being spent on arms even though a famine has been declared

At least half of the country's budget is being spent on arms even though a famine has been declared A US-drafted arms embargo was voted down by the UN Security Council in December, though targeted sanctions have been adopted

UN sanctions monitors said 97 per cent of South Sudan's known revenue comes from oil sales, a significant portion of which is now forward oil sales, and that at least half of the budget — "likely substantially more" — is devoted to security.

"Revenue from forward oil sales totalled approximately $US243 million ($316 million) between late March and late October 2016," the panel of UN monitors said in the report to the UN Security Council, seen by a reporter on Thursday.

"Despite the scale and scope of the political, humanitarian, and economic crises, the panel continues to uncover evidence of the ongoing procurement of weapons by the … Government for the SPLA (South Sudanese army), the National Security Service, and other associated forces and militias," the report said.

The United Nations has declared a famine in some parts of the world's youngest country, where nearly half its population — some 5.5 million people — face food shortages.

A civil war erupted in 2013 when Mr Kiir, an ethnic Dinka, fired his deputy Riek Machar, a Nuer, who has fled and is now in South Africa.

"The bulk of evidence suggests that the famine … has resulted from protracted conflict and, in particular, the cumulative toll of military operations undertaken by the SPLM/A in government in southern Unity state; denial of humanitarian access, primarily by the SPLM/A in government; and population displacement resulting from the war," the report said.

At least one quarter of South Sudanese have been displaced since 2013, according to the United Nations, which has warned that famines crippling the newly-established nation and several nearby countries represent the worst humanitarian crisis since 1945.

The annual report of the sanctions monitors to the 15-member Security Council comes ahead of a ministerial meeting of the body on South Sudan next Thursday, which is due to be chaired by British foreign secretary Boris Johnson.

While the previous US administration of president Barack Obama was heavily involved in the birth of South Sudan, which gained independence from Sudan in 2011, and led Security Council efforts to try to end the civil war, the policy of new US President Donald Trump toward the African state is unclear.

In December, the Security Council failed to adopt a US-drafted resolution to impose an arms embargo and further sanctions on South Sudan despite warnings by UN officials of a possible genocide.

The UN monitors again recommended in their report that the council impose an arms embargo on South Sudan.

At least one quarter of South Sudan's population have been displaced, according to the UN. ( IOM: Bannon via UN News Centre )

China has a significant peacekeeping force in South Sudan and major investments in oil.

The London-based Conflict Armed Research group has previously documented Chinese rifles and ammunition airdropped to SPLA forces.

In March 2015 a targeted sanctions regime for South Sudan was set up by the Security Council, which has blacklisted six generals — three from each side of the conflict — by subjecting them to an asset freeze and travel ban.

The UN monitors said all parties to the conflict continue to commit widespread human rights violations "with near complete impunity and a lack of any credible effort to prevent these violations or to punish the perpetrators".

UN peacekeepers have been in South Sudan since 2011.

Reuters