At least 1,000 refugees are arriving daily in South Sudan's Upper Nile state, fleeing conflict in Blue Nile state across the border, according to aid agencies.
The UN Refugee Agency, UNHCR, says more than 23,000 people have been registered at the Doro refugee camp, about 40km from the border with Sudan, and it is planning new sites as thousands more are expected.
"We're starting up a second site in Jammam [about 65km west of Doro] as Doro is reaching its maximum capacity [of 25,000] and maybe a third when we assess how many people are coming," said Mireille Girard, UNHCR's South Sudan representative.
Only a few aid agencies are in Doro to tend to hundreds of refugees arriving with little or nothing, fleeing aerial bombardment in Blue Nile state, where conflict between Sudan government forces and troops formerly loyal to the south is ongoing.
"I ran away from the bombs; when I heard the sound of the Antonov [bomber plane] in our village, I couldn't even eat, I was so scared, so we ran away," Baabi Ombasha, 43, said.
Ombasha said she walked and camped out for a month with other people in a large convoy, with meagre food rations.
"We took a little bit of sorghum but we finished it. Since arriving here, I haven't eaten for two days," Ombasha said, and neither have her 12 children and seven grandchildren. "Some of the children have diarrhoea and some of them have headaches, they were complaining all the way of the pain."
Alex Balla, coordinator for the Relief and Rehabilitation Commission (RRC) state aid agency in Maban County, said in addition to the refugees, the area had received an estimated 16,000 returnees from the north since independence in July.
"We are expecting 30,000 refugees in Doro soon; some are still on the way and they are coming together with the returnees," Balla said.
"It will be very challenging to the host community because they didn't produce [enough harvests] for their own [needs due to floods in August that affected an estimated 80,000 residents]," he said.
"There's a shortage of food in Maban County in general as well. The WFP [World Food Programme] is now bringing the food but the food basket is incomplete. As you see now, people are just distributing beans and salt.
"In Doro some people are getting food and some people are not - they are just eating leaves from the trees," Balla said.
"If you walk around the camp, you see people defecating everywhere and there is no sanitation," Rebecca Nabukwasi, a nurse for Relief International (RI), said. "I'm really worried that there could be outbreaks of diseases."
Based in nearby Bunj to help resettle tens of thousands of returnees from the north in newly independent South Sudan, RI started a mobile clinic in Doro at a UNHCR way station in November.
"Since we started the clinic, we have seen a lot of cases of acute watery diarrhoea and so many malaria cases," Nabukwasi said.
Medical charity Médecins Sans Frontières (MSF) arrived last week to take over the clinic and is also dealing with high levels of malnutrition and respiratory problems.
"Most of them are sleeping outside, without anything to cover them and they don't have enough blankets or food," said MSF clinical officer Robert Maina in a clinic full of mothers and babies being weighed, immunized and treated. "There is malnutrition - we have noticed many cases especially among children under five years and... the elderly."
MSF is planning to build 30 emergency latrines and aid agency OXFAM arrived this week to plan water, sanitation and hygiene facilities and dig more boreholes for overstretched resources.
"The problem is, people are waiting for five or six hours to get water," said Asaad Khadhum, MSF's field coordinator in Doro.
MSF is bringing in a midwife and an assistant after helping with two emergency deliveries.
"In the camp we have noticed there are so many pregnant women," said Maina.
Aid agencies also worry that the level of a large pond outside the camp used for washing and watering livestock has fallen significantly, which means local water levels are rapidly depleting.
OXFAM also plans to provide services to about 2,000 people already camped outside its compound in Jammam and more aid agencies are expected to move in as numbers swell.
"We have 10,000 people in Elfoj on the border and people say there are thousands more near the border," UNHCR's Girard said.
WFP is battling logistical and supply problems to get in enough food and has distributed rations to 10,000 people.
"We're moving food in as fast as possible," said Michelle Iseminger, deputy director of WFP in South Sudan. "It's very difficult because the local places where we get food have been muddy and blocked [and air freight is limited due to several emergencies in South Sudan]," she said.
WFP has enough food in its warehouse for 5,000 people, and food being trucked in from Kenya should arrive by January.
To cope, some families have already started selling the non-food kits distributed by aid agencies in local markets.
"We know it's going to rain but because of the children's hunger we sold the [plastic] sheets we were given; we only eat once a day. Since morning we've only eaten porridge," said Gadia Mani, whose family has been in Doro for three weeks.
The 10-member family only has two goat kids left to sell and Mani's 28-year-old daughter Raja Dofalla is eight months pregnant with her second child.
"We managed to bring a little food but we finished it on the way," Dofalla said.
"I'm eating nothing; we are just sharing what we have... when I was in my home I was eating meat and dairy, here I just need food," Dofalla said, nibbling on roasted maize and a spiny cactus fruit to which the family is turning in desperation.