The US is providing $180m (£107m) in emergency aid to help address the food crisis in South Sudan, US National Security Adviser Susan Rice says.
She said suffering there was caused by rival leaders’ inability “to put their people’s interests above their own”.
UN Security Council envoys are meeting President Salva Kiir and his opponent Riek Machar in the capital, Juba, in a bid to end the eight-month conflict.
The UN has described South Sudan’s food crisis as “the worst in the world”.
More than a million people have fled their homes since fighting erupted between different factions of South Sudan’s ruling party last December.
Thousands have died in the conflict that started as a political dispute between President Kiir and his ex-deputy Mr Machar, but has since escalated into ethnic violence.
“The scale of the suffering and humanitarian need there is shocking, and the threat of famine is real,” Ms Rice said in a statement on Tuesday.
She said President Kiir and Mr Machar “must immediately assume their responsibilities to the South Sudanese to prevent further needless suffering”.
The announcement came as UN Security Council envoys arrived in Juba to meet with the rival leaders on a two-day mission to end the crisis.
The envoys have reportedly warned both men they could face sanctions if the civil war does not stop, according to AFP news agency.
“The council has made it very clear that it is prepared to impose consequences if there continue to be spoilers, if there continue to be people carrying out gross violations of human rights,” the agency quoted US ambassador to the UN Samantha Power as saying.
Western governments and international aid organisations have condemned the warring sides for failing to meet a regional deadline set earlier this week to end the conflict.
US Secretary of State John Kerry accused both camps of failing to commit to the peace process, after the 10 August deadline for the government and opposition to form a transitional government passed without results.
“Deadlines keep passing and innocent people keep dying,” he said in a statement on Monday.
A truce agreed in January also failed to bring peace, as fighting resumed soon after it was signed.
The Care International charity has warned that the failure to reach a deal means South Sudan’s “severe man-made food crisis could reach catastrophic levels”.
It said almost four million people are now suffering from a food crisis, after months of fighting has prevented farmers from planting or harvesting crops.
Humanitarian aid organisations have faced hurdles when trying to deliver aid and assistance to those who need it, especially with the onset of the rainy season.
Several aid workers have been killed this month and many remain missing.