The crisis engulfing South Sudan is greater than those endured by Darfur or the Central African Republic, according to a senior UN diplomat who says the world urgently needs to donate at least another $500m (£298m) if the country's slide into humanitarian disaster and famine is to be halted.

Thousands of people have been slaughtered, more than a million displaced and almost five million are in dire need of humanitarian assistance as a result of ethnic violence in the world's youngest nation.

Aid and development agencies are warning of a possible future famine in three violence-ridden states. Toby Lanzer, the deputy special representative of the UN secretary general in South Sudan, said the situation was the most desperate he had seen.

"What we are facing is a crisis or an emergency far bigger than I have ever had to deal with – and I used to work in Darfur and the Central African Republic," he said. "It is simply no exaggeration to say that we are currently facing one of the most severe tests that the international aid community has ever faced."

His bleak assessment of the crisis echoes that of the UN secretary general, Ban Ki-moon, who warned last week that if the conflict continued half of South Sudan's 12 million people would be either "displaced internally, refugees abroad, starving or dead by the year's end".

The violence – which began in December after President Salva Kiir accused his vice-president, Riek Machar, of plotting to overthrow him – has split the country along ethnic lines, sparking clashes between Kiir's Dinkas and Machar's Nuers.

Machar, who has fled the capital, Juba, has denied the accusations, claiming he has been made a scapegoat for the violence that broke out when rival factions in South Sudan's army turned on each other. The fighting has left tens of thousands of people dead or wounded.

In a report last week, the UN laid out gross human rights violations – including ethnic-based killings, rapes, revenge attacks, and the deliberate targeting of civilians – on what it termed "a massive scale".

Navi Pillay, the UN high commissioner for human rights and former president of the international criminal tribunal for Rwanda, said she recognised in the report "many of the precursors of genocide".

The humanitarian situation is deteriorating rapidly, the World Food programme (WFP) and the UN Office for the Coordination of Humanitarian Affairs have warned. The former predicts that 3.9 million people in South Sudan will face emergency or crisis levels of food insecurity between June and August, while the latter estimates that 4.9 million people are in immediate need of humanitarian aid.

The fighting has displaced almost 1.3 million people, with more than 310,000 fleeing into Ethiopia, Kenya, Sudan and Uganda. To make matters worse, the international aid system is under serious pressure because of the violence in Syria, where 9.3 million people require assistance, and the Central African Republic, where 2.5 million people need help.

Lanzer, who will travel to Oslo next week to address a two-day humanitarian pledging conference on South Sudan, said he would ask donors for a rapid initial injection of $500m to help meet the needs of the country and to "stop things spiralling in the wrong direction". He said the funds were needed in addition to the $540m raised so far this year.

"The economy is shattered, cities have been destroyed, traders have left en masse and no longer want to work in South Sudan because of insecurity," Lanzer said. "People have been unable to reach their fields during the crucial months of April and May to cultivate and they have been unable to tend to their sheep, their cattle and their goats. This is a largely subsistence farming society and, if you cannot cultivate and you cannot tend to your livestock, you're finished."

Malnutrition levels had surged, he added, with acute malnutrition at 38% in parts of the country. Lanzer said that while he was sympathetic to the immense strain the international donor community was under because of the crises in Syria and the Central African Republic, the world had to find additional money to help the people of South Sudan in "their greatest hour of need".

"I know that we need much, much more – but in a cash-strapped donor environment I cannot be greedy. I'm looking for nuts and bolts, bare-bones emergency relief response to the current needs and preventing a famine," he said.

A survey by the South Sudanese government and aid agencies, including the UN Food and Agriculture Organisation, the WFP and Save the Children, found that food security had dropped to alarming levels in the three conflict-affected states of Jonglei, Unity and Upper Nile, and that famine could become a serious risk this year if adequate humanitarian assistance did not get to those in need.

Oxfam said South Sudan's crisis was reaching a tipping point with the onset of the rainy season. "We either act now or millions will pay the price," said Mark Goldring, chief executive of Oxfam GB. "We face a mammoth task of getting massive levels of aid to people at the worst time of the year when rains make many areas hard to reach and turn roads into rivers of mud."

The longer people had to wait to sow their crops, rebuild their homes and get their children back to school, he said, the worse things would get. "We need a massive and rapid global surge in aid to prevent catastrophic levels of hunger," he said. "We cannot afford to wait, we cannot afford to fail."

Greg Barrow, a spokesman for the WFP, said securing an enduring truce was vital to getting supplies to the two million people in urgent need of food assistance – and to staving off famine.

"As long as we have that access, there's a good chance that this can be avoided," he said. "While everyone in the humanitarian community is deeply concerned about the potential for problems down the line, there's a sense that if conditions are such that we get everything we need in terms of access, we're going to be in a much better position to meet the needs of the vast numbers of people who have been displaced by the conflict."

But Barrow pointed out that it was not just a question of access. The WFP was facing a $241m shortfall in its funding for operations in South Sudan over the next six months, he said.

The British government, which has allocated about £20m to help meet humanitarian needs in the country, is urging the world to give generously in Norway.

"Humanitarian agencies operating in the country remain woefully underfunded," said the international development minister, Lynne Featherstone. "At next week's Oslo conference the international community needs to step up and scale up support for the world's newest country, to stop this terrible crisis from getting even worse. It is also vital that the government of South Sudan sets out practical steps to ensure the unfettered delivery of aid."

Hopes of a pause in hostilities are fading. A second ceasefire brokered in neighbouring Ethiopia last weekend lasted only a few hours before fighting resumed, with each side blaming the other. Agence France-Presse reported heavy fighting on Thursday in the oil-rich Upper Nile, which has been pumping almost all of South Sudan's crude since intense battles shut most fields in the other main oil-producing area in Unity state.