More aid workers have been killed in Sudan's Darfur region in the last two weeks than in the past two years
The UN's Office for the Co-ordination of Humanitarian Affairs says access to people in need in Darfur is at its lowest level since the conflict began.
Aid agencies say the situation has got worse since the May peace accord.
More than 2m Sudanese have fled their homes and tens of thousands have been killed in the three-year conflict.
"The level of violence being faced by humanitarian workers in Darfur is unprecedented," Manuel da Silva, UN humanitarian co-ordinator in Sudan said in a statement released by the UN.
"This is completely unacceptable."
Eight humanitarian workers died in the line of duty during July.
Four international agencies working in Darfur - Care, International Rescue Committee, Oxfam International and World Vision - said the situation had worsened since an accord signed in May by the Sudanese government and one faction of the Sudanese Liberation Movement.
On Monday, the leader of that faction, Minni Minnawi, started his work as a special advisor to President Omar al-Bashir.
"Since the signing of the agreement, Darfur has become increasingly tense and violent, which has led to the tragic deaths of far too many civilians and aid workers.
"A full and comprehensive ceasefire must be implemented immediately," said Paul Smith-Lomas, Oxfam's regional director in Sudan.
A statement issued by the four agencies also warned that increasing insecurity was damaging their ability to reach people in need, with "potentially disastrous" consequences.
In one incident in the Zalinge area in the west of Darfur, a mob beat three aid workers to death, after which international organisations suspended activities in displaced people's camps.
The UN also reports an increase in the number of ambushes and hijackings of vehicles belonging to non-governmental organisations.
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