South Sudan's government is blocking food aid and restricting United Nations peacekeepers, according to the UN.
After a two-day visit, UN humanitarian chief Stephen O'Brien said on Monday that obstacles to humanitarian assistance included active hostility, access denials and bureaucratic impediments.
"People have been displaced, brutalised and raped. They have been attacked when they sought out assistance. This must stop, and it must stop now," O'Brien said in a statement.
Obtained by the AP news agency, an internal report to Security Council members from Antonio Guterres, the UN secretary-general, criticised the government for "the destruction of all the social fabric in all parts of the country" and listed "outrageous" examples of belligerence by South Sudan's security forces.
Sent on February 13, Guterres' letter said UN peacekeepers were recently prevented from verifying allegations of killings or arbitrary arrests of civilians, including in the town of Yei.
In late February, armed groups and members of the local community looted the compound and warehouse of Save the Children, a humanitarian organisation, in the northern Jonglei area.
The group 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," Peter Walsh, South Sudan director for Save the Children, said 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."
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But Taban Deng Gai, the country's first vice president, told members of the UN Human Rights Council on February 27 that the government has improved security and taken steps to hold violators of human rights 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," Gai said in the 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 producer of migrants, as more than three million people have either fled the country or become internally displaced, according to the UN.
Source: News agencies