Keira Knightley has spoken about the "terrible trauma" she witnessed when she travelled to South Sudan.
The actress saw the desperate plight of families struggling to survive when she visited the country last month.
"I had no idea what to expect when I arrived in South Sudan, but what I saw and heard was worse than I could have ever imagined," Keira said.
"All of those I met were suffering a terrible trauma unbearable to comprehend. I spoke to women who have lost their husbands and children within months of one another.
"They are now alone trying desperately to get through each day struggling to provide enough food and water to keep them and their remaining children alive."
Keira travelled with Oxfam to Bor camp, Jonglei State, to meet families in one of the most affected areas of the war-torn country.
After seeing people living in atrocious conditions in an area where poor sanitation has already cost lives, she called on the public to support Oxfam's appeal for South Sudan.
Oxfam chief executive Mark Goldring said: "We are grateful to Keira for travelling with Oxfam to see for herself the devastating nature of this crisis, and the terrifying situation facing the people of South Sudan - just three years after their nation's independence.
"This is a country at tipping point. We will be staring into the abyss and fail to avert a famine if funds do not start arriving soon to help the people of South Sudan at risk of starvation, disease and violence."
Conflict broke out in South Sudan last year, and more than 1.5 million people have been forced to leave their homes in search of safety.
Nearly four million - a third of the country's population - are at risk of severe hunger and the UN has warned that if the aid effort does not increase 50,000 children could die from malnutrition.