A month of fresh violence in the Sudanese region of Darfur has driven more than 200,000 people from their homes, the United Nations has said.
Insecurity is also blocking aid to the 1.5 million displaced people inside the region, according to the UN's humanitarian co-ordinator for Sudan.
Manuel Aranda Da Silva said robberies of aid workers were on the rise.
The UN has threatened Sudan with sanctions if it fails to halt violence in Darfur, scene of a recent rebellion.
UN Secretary General Kofi Annan appointed a panel this month to investigate reports of genocide in Darfur, a remote, arid region the size of France where some 50,000 people have died in the past 18 months.
The government is accused of failing to rein in the Janjaweed, an ethnic Arab militia blamed for killing thousands of black African civilians and emptying villages as part of a campaign against local rebels.
The deaths in a landmine blast of two aid workers, a Briton and his Sudanese colleague, have highlighted the dangers facing humanitarian agencies.
Aid efforts a few months ago were hampered by logistical problems but now insecurity seems to be the chief obstacle, Mr Da Silva told Reuters news agency in Khartoum.
"Security is probably becoming the main constraint to the delivering of humanitarian assistance in Darfur."
There had, he added, been a "negative trend" of armed robberies against
humanitarian workers in Darfur in the past three weeks.
"This is a general trend that we are worried about," he said.
Despite a fragile ceasefire signed between the government and the rebels in April, ethnic conflict, attacks on civilians and clashes between Sudanese armed forces and rebels have all increased, Mr Da Silva said.
The UN panel appointed by Mr Annan has been given three months to report back on the situation
The US has already spoken of genocide in Darfur while human rights organisations have said attacks on civilians there amount to war crimes.