Fourteen UN aid agencies working in Sudan's troubled Darfur region have warned that their relief operations will collapse unless security improves.
Malnutrition is on the rise again in Sudan's Darfur region
Humanitarian workers, they said, are "holding the line" for the survival and protection of millions in Darfur.
But they need "solid guarantees" of security from all the parties involved in the conflict to be able to continue.
Violence in the western region of Sudan has claimed more than 200,000 lives and led 2.5 million to abandon their homes.
"The UN and its humanitarian partners have effectively been holding the line for the survival and protection of millions," the UN agencies said in the joint statement.
"That line cannot be held much longer."
The UN said that their humanitarian operations in Darfur, which employ almost 14,000 aid workers and costs more than $1bn (£0.5bn, 0.7bn euros), had saved hundreds of thousands of lives since it began in mid-2004.
But the agencies, which include the World Food Programme (WFP), Unicef and the World Health Organisation, said that "malnutrition rates are edging perilously close to the emergency threshold."
Security fears led to the distribution of double food rations in some areas in the month of December, and also prevented some 47,000 people in need being reached, WFP said.
The agencies said that the primary victims of Darfur's conflict are civilians who suffer due to "repeated military attacks, shifting frontlines and fragmentation of armed groups."
A total of 12 aid workers have been killed in the last six months, the statement said - more than the number in the previous two years combined.
Also in the last six months, 30 compounds operated by relief groups, including non-governmental organisations and charities, were directly attacked by armed groups.
The Sudanese government says the instabilty and number of deaths in Darfur have been exaggerated by the west.