Sharply deteriorating security in the Darfur region of Sudan has led to the withdrawal of 250 relief workers.
Millions of Darfur people depend on humanitarian aid
Aid workers face "unprecedented difficulties" because of military activity and direct violence against them, a statement by six agencies says.
They said the withdrawal of staff affects the provision of aid to nearly 500,000 displaced people in Darfur.
It is estimated about 200,000 people have died in Darfur since 2003. Many more have been driven from their homes.
Friday's statement was signed by Oxfam, the International Rescue Committee, Goal, Concern, World Vision and the Norwegian Refugee Council.
They say military activity, banditry and violence against humanitarian workers have increased across Darfur since early December.
The latest upsurge has led to the temporary withdrawal in the last 10 days of staff serving a total of 488,000 people, according to the agencies.
"If the deterioration is allowed to continue, the impact on civilians could be devastating," says Paul Smith-Lomas, Regional Director for Oxfam.
"With new displacements and attacks, the presence of aid agencies is more important than ever. Yet every day brings one huge blow after another to aid efforts," he adds.
The aid agencies say their workers are being attacked in some cases, vehicles stolen and stocks of aid pillaged.
The violence now reaches inside refugee camps which the aid agencies' statement says are now "increasingly rife with weapons and armed groups".
The agencies renewed an appeal for all sides to observe a ceasefire to allow a full resumption of aid activities.
Blame cannot be attributed to just one group, says the statement, which calls for an immediate ceasefire.
The BBC's Jonah Fisher in Khartoum says the call is likely to fall on deaf ears as the Sudanese government and its partners in the Janjaweed militia appear to be stepping up their activities.
From being elusive figures patrolling Darfur's barren landscape, the militia are now present in the centre of many key towns, our correspondent says.
The violence has intensified despite the presence of some 7,000 African Union peacekeepers.
Sudan rejects plans for the United Nations to take control and increase the number of peacekeepers to 20,000.
The conflict began in early 2003, when a rebellion by local groups triggered a counter-offensive by the army and government-backed Arab militias.