South Sudan's government is blocking desperately needed food aid and restricting United Nations peacekeepers, according to the UN.
An internal report from UN secretary-general Antonio Guterres to members of the Security Council obtained by the Associated Press shows the daunting conditions faced by the international community as it tries to combat a catastrophe in the troubled East African nation.
It singles out South Sudan's government for "the destruction of all the social fabric in all parts of the country" and lists "outrageous" examples of belligerence by South Sudan's security forces.
The UN humanitarian chief, Stephen O'Brien, said South Sudan is impeding humanitarian assistance, following a two-day visit to the country over the weekend.
"People have been displaced, brutalised and raped. They have been attacked when they sought out assistance. This must stop, and it must stop now," Mr O'Brien said in a statement.
At least 50,000 people have died in South Sudan's civil war, which began in December 2013 as a result of a struggle for power between President Salva Kiir and former Vice President Riek Machar.
An estimated 100,000 people are experiencing famine, and another one million people are on the brink of starvation, South Sudan's government and UN agencies said in late February.
South Sudan is now Africa's largest migrant crisis as more than three million people have either fled the country or become internally displaced, according to the UN.
The impact of this ongoing conflict and violence has reached disastrous proportions for civilians, Mr Guterres said in the internal letter.
The UN Security Council decided in August to send an additional 4,000 peacekeepers to South Sudan, but the government has delayed the arrival of the extra troops.
Some progress on sending the extra troops has recently been made, however, and the deployment of an advance contingent of Rwandan troops is being finalised, said Mr Guterres' report.
The letter, sent to the Security Council on February 13, listed several incidents in which he said government forces hindered the UN's peacekeeping and humanitarian work.
But South Sudan's First Vice President Taban Deng Gai told members of the UN Human Rights Council on February 27 hat the government has improved security and taken steps to hold violators of rights abuses accountable, according to a statement obtained by AP.
He said the government does not have enough resources to demobilise armed groups, and asked for more military funding.
"I can state with confidence that the notion of a looming genocide and possible ethnic cleansing is fading away as we continue with these demonstrations of our commitment to harmoniously live together," Mr Gai said in the statement.
His optimistic portrait of South Sudan is markedly different from the one UN officials provide.
UN peacekeepers were recently prevented from verifying allegations of government forces killing or arbitrarily arresting civilians, including in the troubled town of Yei, according to the internal letter from the secretary-general.
In late February, armed groups and members of the local community looted the compound and warehouse of Save the Children in the northern Jonglei area.
The organisation was the only distributor of food aid in the area, which is on the brink of famine.
"This is the most extreme act by the very people we are trying to help," said Peter Walsh, South Sudan director for Save the Children in a statement.
"It is critical that parties to the conflict provide unimpeded humanitarian access to the affected community to avoid famine becoming their death sentence."