Famine has been declared in two counties of South Sudan, according to an announcement by the county's government and three UN agencies.
The calamity is the result of prolonged civil war and an entrenched economic crisis that has devastated the war-torn East African nation, the announcement said.
The official classification of famine highlights the human suffering caused by South Sudan's three-year civil war and even as it is declared President Salva Kiir's government is blocking food aid to some areas, according to UN officials.
More than 100,000 people in two counties of Unity state are experiencing famine and there are fears that the famine will spread as an additional one million South Sudanese are on the brink of starvation, said the announcement.
"Our worst fears have been realised," said Serge Tissot, head of the Food and Agriculture Organisation in South Sudan.
He said the war has disrupted the otherwise fertile country, causing civilians to rely on "whatever plants they can find and fish they can catch".
Roughly 5.5 million people, or about 50% of South Sudan's population, are expected to be severely food insecure and at risk of death in the coming months, said the report.
Nearly three-quarters of all households in the country suffer from inadequate food, it added.
If food aid does not reach children urgently "many of them will die", said Jeremy Hopkins, head of the UN children's agency in South Sudan.
Over 250,000 children are severely malnourished, Mr Hopkins said, meaning they are at risk of death.
It is not the first time South Sudan has experienced starvation.
When it fought for independence from Sudan in 1998, the territory suffered from a famine spurred by civil war.
Anywhere from 70,000 to several hundred thousand people died during that famine.
But Monday's declaration of starvation is solely South Sudan's creation, and a UN official blamed the country's politicians for the humanitarian crisis.
"This famine is man-made," said Joyce Luma, head of the World Food Programme in South Sudan.
"There is only so much that humanitarian assistance can achieve in the absence of meaningful peace and security."
Perhaps nowhere else has civil war caused such a drastic decline in South Sudan's food security than in Central Equatoria state, according to the report.
Traditionally South Sudan's breadbasket, Central Equatoria has been hit by fighting and ethnically targeted killings that began in July 2016 and have displaced over half a million residents and disrupted agricultural production.
As a result, more than a third of Central Equatoria's population is now facing crisis or emergency levels of hunger, according to the report.
Although it is not as significant as the effects of war and inflation, some of South Sudan's hunger crisis is the direct result of the government's action.
South Sudanese government officials have blocked or placed constraints on the delivery of food aid to areas of the country, according to a UN official.
On Monday, the UN agencies said unimpeded humanitarian access "is urgently needed."
Tens of thousands of people have died since civil war broke out in December 2013, and the UN warns South Sudan is at risk of genocide.
Since fighting in the capital of Juba killed hundreds of people in July, the war has uprooted more than three million people.