According to a UN spokeswoman in Khartoum on Wednesday, the recent upsurge in violence that left three African Union (AU) soldiers dead and the brief abduction of nearly 40 other AU personnel by rebel forces are major causes for concern.
"Almost two-thirds of the area of operation of the humanitarian community in South Darfur is considered hazardous for the safety of humanitarian personnel and has been declared no-go areas," Radhia Achouri said.
She added that tensions in and around Geneina, the capital of West Darfur state, had led to a deterioration in security conditions, prompting the United Nations to issue a travel warning for aid workers.
The United Nations mission "has declared all roads leading out of the town to be restricted in view of armed clashes north, west and south of the town, in addition to increased banditry along all roads", Achouri said.
She added that only North Darfur state appeared to be relatively safe.
Humanitarian access to North Darfur, she said, "has been less affected and most areas of operation are accessible".
The United Nations said it continued to receive reports from the Darfur region on incidents of banditry, looting and robbery that sometimes involve firearms and result in casualties.
The UN mission said there have been reports of attacks by nomadic tribesmen on villages in Darfur that the AU was investigating.
"Thousands of civilians are fleeing to IDP [internally displaced persons] camps throughout the region" due to the violence, Achouri noted.
An estimated 180,000 to 300,000 people have died in Darfur since a civil conflict between rebels and government-backed nomadic militias erupted in February 2003, with 2.6 million civilians left homeless.