About 100,000 people face immediate starvation, with millions more on the brink of famine. The UN warns that nearly half of South Sudan's population will be without food by July if no action is taken.
What caused the famine?
South Sudan has been embroiled in a civil war since 2013.
The fighting has uprooted more than three million people and prevented farmers from harvesting their crops, raising the risk that not enough food would be available in 2018, a UN report released this week said.
With the collapsed economy, hyperinflation soared more than 800 percent last year, increasing the price of imported food products and making it unaffordable for many.
The fighting may put 4.9 million more people at risk for starvation in the next two months. The number could rise to 5.5 million by July, chairman of South Sudan's National Bureau of Statistics Isaiah Chol Aruai said.
Drought in parts of the country has exacerbated the crisis.
What is a famine?
During famine, there is a severe food shortage.
The UN said a food crisis becomes famine when at least 20 percent of households face extreme food shortages, acute malnutrition rates exceed 30 percent and two or more people per 10,000 are dying per day.
What is being done?
South Sudan has been receiving humanitarian and food assistance from international relief organisations including the UN.
Delivering aid is difficult as the conflict prevents humanitarian workers from accessing affected areas. UN agencies have called for urgent intervention.
South Sudan's President Salva Kiir said the government "will ensure that all humanitarian and development organisations have unimpeded access to needy populations across the country."
But aid agencies say they need to be sure about the security of their workers if they are to go into the conflict areas.
They also want protection for the air convoys that are regularly looted.
How did it get to this point?
"This famine is man-made," World Food Programme Country Director Joyce Luma said.
Humanitarian organisations say the famine was a result of ongoing conflict and economic decline.
Six years after independence in 2011, South Sudan remains one of the least developed countries. The war has not allowed the country to exploit its oil reserves, one of the largest in Africa.
The most recent upsurge in violence followed a failed ceasefire in July 2016.
The famine was declared in areas that have seen some of the worst fighting between the government and rebel forces and could further complicate the situation.
"There is only so much that humanitarian assistance can achieve in the absence of meaningful peace and security," Luma said.
How will the famine affect neighbouring countries?
Hundreds of thousands of people are fleeing the war and hunger. The United Nations High Commissioner for Refugees (UNHCR) last week said that 1.5 million people have crossed into neighbouring countries including Uganda, Ethiopia and Sudan.
About a third of refugees go to Uganda.
This is the largest refugee crisis in Africa and the third largest in the world after Syria and Afghanistan, UNHCR said.
The massive influx is putting a burden on its neighbours, who themselves struggle with basic infrastructure including health and housing facilities.