Europe is growing increasingly frustrated by "relentlessly deteriorating" humanitarian crisis in South Sudan, an European Union official has said.

Having contributed well over $1 billion worth of relief since the start of conflict in Juba 18 months ago, a handful of rich countries are balking at requests for an additional billion dollars in aid for the war-torn country.

They see supplies being blocked or stolen as antagonists ignore demands for a halt to fighting that has killed thousands of civilians and driven almost a quarter of the country's 12 million population from their homes.

While urging an end to the "senseless violence," the United States last week pledged $133 million in new humanitarian assistance to South Sudan.

Nine European countries, along with Australia and the European Commission announced at a conference in Geneva that they would provide a total of $142 million in added relief.

China and oil-rich Gulf states were conspicuous in their non-participation in the South Sudan humanitarian fundraising event in Geneva.

The projected overall infusion of $275 million will still leave funding roughly $500 million short of the amount the United Nations estimates is needed this year to respond adequately to the human calamity in the country.

However, there is no guarantee that the promises made in Geneva will actually be kept. About $200 million of $529 million pledged at an international aid meeting in Nairobi in February has yet to take the form of on-the-ground relief in South Sudan.

"It's a drop in the bucket. It will be insufficient," Kurt Tjossem, East Africa coordinator for the International Rescue Committee, said in regard to the aid promised at last week's Geneva conference.

"Donors are understandably frustrated," added Zlatko Gegic, director of Oxfam's programme in South Sudan. "If the war continues, there will be little relief for the people of South Sudan. The Geneva conference comes at a time of other severe, simultaneous emergencies around the world which are competing for similar aid resources."

Globally, the gap between need and funding is also widening.

The UN relief agency calls the international response to its most recent appeal "worryingly disappointing."

Only $4.8 billion has been committed out of a total of more than $18 billion sought for humanitarian emergencies in Syria, Yemen, Ukraine and Nepal, as well as in Somalia, the Central African Republic, Democratic Republic of Congo and South Sudan.

“This raises questions about our ability to continue to meet affected people’s needs, especially in protracted situations where 80 per cent of our work now takes place, and where we are asked to stay longer and to do more," said Stephen O'Brien, UN under-secretary-general for humanitarian affairs.

The number of South Sudanese described as "severely food insecure" is expected to increase in the coming weeks from a current total of 4.6 million, or 40 per cent of the country's total population.

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The approaching rainy season will bring heightened risk of malaria, cholera and dysentery, aid workers warn. They note that thousands of families recently uprooted by violence in Unity and Upper Nile states are hiding in swamps where they are beyond the reach of relief providers.

Children are also being butchered in those areas, Unicef director Anthony Lake pointed out last week.

"Survivors report that boys have been castrated and left to bleed to death," he said. "Girls as young as eight have been gang-raped and murdered. Children have been tied together before their attackers slit their throats. Others have been thrown into burning buildings."

Food prices are meanwhile soaring in parts of the country relatively unscathed by the war. Growing scarcity resulting from disruptions of farming has driven up the price of staples by 40 per cent since January.

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South Sudan is "on the verge of economic collapse,” the country's UN humanitarian coordinator, Toby Lanzer, warned last month. The Juba government responded to that statement by ordering Mr Lanzer to leave the country.

Nearly 2 million South Sudanese are listed as "internally displaced," with 135,000 of them having sought shelter at UN compounds in the country. And those civilian-protection sites are increasingly being targeted by combatants, South Sudan UN force commander Lieutenant-General Yohannes Gebremeskel Tesfamariam told the Security Council last week.

Another 600,000 South Sudanese have fled their homeland since the start of fighting in December 2013. Ethiopia is hosting 209,000 of these refugees, with 153,000 having reached Uganda, 157,000 in Sudan, and 46,000 in Kenya.

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About 10 per cent of the funding needed to assist these refugees has so far been provided by donors, the UN reports.

"Donors don't say they're fatigued or indifferent, but they are frustrated there has been so little movement toward peace," East Africa aid coordinator Tjossem commented in regard to the growing shortfall in humanitarian funding for South Sudan.