The civil war in South Sudan has left people so hungry and desperate for relief that they are even fleeing across the border into Darfur, a long-troubled region of famine and suffering in neighbouring Sudan, according to the United Nations.

About 900,000 South Sudanese are homeless since the conflict erupted in December, and about 195,000 of them have fled as refugees to Uganda, Ethiopia and even into Darfur, the UN humanitarian coordinator for South Sudan Tony Lanzer told the Associated Press on Tuesday.

I never thought I would see people fleeing into Darfur ... It's a very painful thing for the world's youngest country if your people are fleeing. Tony Lanzer, UN humanitarian coordinator for South Sudan

South Sudan broke away from Sudan to become independent in 2011. Sudan's western Darfur region has been gripped by violence since 2003, when rebels took up arms against the government.

"I never thought I would see people fleeing into Darfur," Lanzer said. "It's a very painful thing for the world's youngest country if your people are fleeing."

South Sudan's conflict broke out in December between supporters of ousted Vice President Riek Machar, from the Nuer ethnic group, and the forces of President Salva Kiir, who is an ethnic Dinka.

The two sides agreed to a ceasefire in January, but that agreement does not appear to be holding.

"The single biggest thing I need right now is a ceasefire, to get them help now," Lanzer said.

'Food insecure'

A total of 3.7 million South Sudanese are "food insecure," or unsure of where their next meal will come from, he said, out of a population of about 11 million.

Lanzer is organising donations for international relief aid in the coming weeks during the dry season, when roads are passable.

The World Food Programme hopes to pre-position 146,000 tons of food. By June, during the wet season, supplies would have to be airlifted at far greater cost.

"Now, 90 percent of funds go toward relief, and 10 percent to delivery," Lanzer said. By June, that ratio will have reversed.

Adding to the urgency, people need to sow crops before June but are afraid to go into the fields.

"There will not be a harvest if people do not cultivate. If the violence continues - and there's a high risk of that in some of the key states, which are the most food insecure and which are the most prone to flooding - then the outlook for the humanitarian situation is very dire," he added.