The UN's humanitarian chief, Jan Egeland, has warned that the aid operation in Darfur region is at risk because of threats to aid workers.
Millions of people have been displaced by the conflict
Mr Egeland said killings, rapes and forced displacement were continuing in the region of western Sudan.
As a result, the situation was deteriorating, he told the UN Security Council in a report.
Mr Egeland also warned of the "significant threat" posed by the rebel Lord's Resistance Army in Uganda.
The UN's top relief emergency co-ordinator told the Security Council the humanitarian operation in Darfur had been remarkably effective, despite overwhelming odds.
However, he warned that the terrible reality on the ground was that the killing had not stopped and that for the past three months the situation had deteriorated, with 20,000 more displaced in the last few weeks.
Meanwhile, the US congress has rejected a request by Secretary of State Condoleezza Rice for $50m to help fund African Union peacekeepers in Darfur.
There was no immediate explanation for the reasons behind the decision. The AU says it only has resources for the peacekeepers for a few more months.
Tens of thousands have been killed and two million displaced from their homes since the violence began in early 2003, with pro-government militias attacking villages as part of a campaign against a rebel uprising.
Mr Egeland said that in a worrying new development, the camps for those forced to leave their villages were also being attacked.
He said aid workers were under constant threat and their operations could be completely disrupted at any time.
"We must be acutely aware that all that has been built up by the thousands of relief workers and hundreds of millions of dollars in donor contributions could be destroyed," he said. "We could be on the brink of losing this huge humanitarian operation."
Mr Egeland also discussed the violence in northern Uganda, where for nearly 20 years the rebel Lord's Resistance Army have ambushed villagers and become known for abducting children and forcing them to fight.
Mr Egeland said the LRA was deliberately targeting humanitarian workers, making it difficult for the UN to get into camps for the internally displaced.
He said the LRA was also wreaking havoc in southern Sudan and hampering preparations for the return of refugees there. A small number of fighters, he said, was threatening a huge area and millions of people.