International aid organisers are scheduled to meet in Nairobi on Monday in response to the worsening hunger crisis in war-ravaged South Sudan.

The United Nations relief coordinator, officials of East African governments and a US assistant secretary of state are among those taking part in the "High-Level Event on the Humanitarian Crisis in South Sudan and its Impact in the Region."

According to the UN, donors must provide $600 million this month to enable aid to reach parts of South Sudan where household food stocks are expected to run out soon.

"The humanitarian outlook is grim," the UN observes.

A total of $1.8 billion is said to be needed this year to help feed nearly nine million South Sudanese. An additional $809 million is being sought to assist close to half-a-million refugees who have fled South Sudan.

The day-long meeting at the UN complex in the Kenyan capital is taking place against a backdrop of stalled negotiations on ending the 14-month-long civil war in South Sudan that has taken an estimated 50,000 lives.

World leaders have called repeatedly for an end to the fighting. On Friday, a South Sudan-focused "troika" consisting of the US, United Kingdom and Norway urged the warring sides to reach a peace agreement by March 5.

The three countries said they are "profoundly disappointed" by the ongoing failure to resolve the conflict.

The fate of millions of hungry South Sudanese rests on finding a way out of the impasse, donors suggest.

"Humanitarian aid can only be effective if the government of South Sudan, opposition forces and all other parties to the conflict stop fighting and remove obstacles to the delivery of life-saving assistance," the US State Department said on Friday.

Valerie Amos, the UN under-secretary-general for humanitarian affairs, will be joined at the Nairobi session by Anne Richard, head of the US State Department's Bureau of Population, Refugees, and Migration.

The event is being co-sponsored by the UN and by East Africa's Intergovermental Authority on Development.