"We have 2.5 million people urgently in need of help with food," Valerie Amos, UN assistant secretary-general for humanitarian affairs and humanitarian relief coordination, told a news conference in Juba.
"There is a woman, a man and a child behind every statistic when we talk of the numbers here in South Sudan," she said.
Amos arrived in the country on Friday to assess the impact of the ongoing crisis, which began in December of 2013, on the people and aid work.
She has since met with internally displaced people in Juba and Ayod County in Jonglei State, two of the areas hardest-hit by the conflict.
Amos also held talks with President Salva Kiir and top government officials on the humanitarian and economic situation.
"There are thousands of children suffering from malnutrition; the threat of hunger and disease is real," she said.
South Sudan has been shaken by violence since late 2013, when President Kiir accused Riek Machar, his sacked vice-president, of leading a failed coup attempt against his regime.
"It's heartrending to see the suffering of the people," said Amos.
According to the UN Office for the Coordination of Humanitarian Affairs, some two million South Sudanese have fled their homes, with 490,000 fleeing the country altogether.
Nearly 6.4 million South Sudanese, meanwhile, have been left food insecure. Aid agencies say they need $1.8 billion to implement the Strategic Response Plan for 2015 for South Sudan, along with an additional $810 million for operations in countries hosting South Sudanese refugees.
"We absolutely recognize that – even if a peace deal was signed tomorrow – there would be an urgent need to continue helping the people," said Amos.
"Right now, we are looking at the prepositioning of supplies before the rainy season," she added.
Kiir and Machar recently signed a power-sharing agreement in Ethiopian capital Addis Ababa in hopes of ending the crisis.
According to Amos, the South Sudanese people are physically and emotionally exhausted from the more-than-year-long conflict.
"People are desperate for peace and tired of living in fear," she said. "Many have had to flee [their homes] several times."
She said people were worried their children were being recruited by armed groups and that sexual violence was becoming rife.
"People I spoke to want to live in safety, peace and stability," the UN official asserted. "People need peace, and they need it now."
For his part, Forest Whitaker, U.S. actor and UNESCO's special envoy for peace and reconciliation, lamented the continued suffering of children as a result of the crisis.
"I am deeply troubled by millions of children in the internally-displaced persons camps," he told the same press conference.
"I welcome the release of 250 children by the [South Sudan Democratic Army's] Cobra Faction, but there are still thousands of children recruited in the conflict," Whitaker noted.
UNICEF and its partners recently secured the release of nearly 3,000 child soldiers in war-torn South Sudan, 280 of whom were discharged in late January by David Yau Yau's Cobra Faction of the South Sudan Democratic Army.
According to UNICEF, 12,000 children – mostly boys – were recruited and used as soldiers by armed forces and groups in South Sudan in 2014.
"If another conflict breaks out in five years or so, these children will be the first ones to be recruited back," Whitaker warned. "Education is key for these children."
Discharged child soldiers are being provided with basic healthcare, protection, food, water and clothing. They will soon also have access to education and UNICEF will attempt to reunite them with their families.
The UN children's fund has put the cost of reintegrating each former child soldier at some $2,330 for two years.