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.