Mortality rates in a refugee camp in South Sudan are nearly double the threshold for an emergency, Medecins Sans Frontieres has warned.
This means that about eight children are dying every day in the camp, which houses 40,000 people from the conflict in Sudan's Blue Nile state.
The medical charity said people were dying of preventable diseases because of "horrific living conditions".
Latrines have overflowed, contaminating water sources, because of heavy rains.
South Sudan seceded from Sudan exactly a year ago next Monday - as part of a deal to end years of civil war.
The aid agency International Rescue Committee says the new nation is marking its first year of independence with development at a "virtual standstill".
'Exposed on flood plain'
Jamam camp is one of three refugee camps in South Sudan's Upper Nile state where about 120,000 people have fled.
"These people have fled terrible violence in Sudan and lost family members during their arduous journeys for safety, and now they are sitting exposed in refugee camps on a flood plain and dying from preventable diseases due to horrific living conditions," Tara Newell, MSF emergency co-ordinator in Jamam, said in a statement.
She said urgent action was needed by the UN refugee agency.
"What's needed is for all agencies involved, led by the UNHCR, to come up with a solution together that can remove these refugees from the health risks associated with the dire living conditions in the camp. We have to proceed with a great sense of urgency."
The UN says it has been trying to move people from unsuitable locations but the pace of new arrivals and shortfall in funding and resources has compounded the challenge.
"It's a very complex and difficult situation and we're working with NGOs such as MSF and other health actors and doing our best," UNHCR's Teresa Ungaro told the BBC's Network Africa programme.
"We're juggling the safety of refugees with trying to identify places with adequate water, with finding sites which are free of landmines," she said.
A third camp in Upper Nile had been opened at the end of May but a sudden influx of 32,000 refugees meant that they had to be given priority and moved from insecure border areas, where people have been bombed in the past, before transferring those in Jamam, she said.
Since last year's secession conflicts have erupted in two of Sudan's border states where communities traditionally allied to the south found themselves north of the border after Juba's independence.
"From the standpoint of improving the quality of life for millions of South Sudanese people, these first 12 months of independence can be written off as a lost year," Susan Purdin, head of IRC's aid programmes in South Sudan, said.
South Sudan's decision to halt oil production amid unresolved disputes with Sudan meant it virtually had no revenue for use in building hospitals, roads and water systems, the aid group said.