Fresh fighting has forced 10,000 people to seek refuge near an African Union camp in the Muhajiriya area of Darfur.
The AU force has been monitoring a widely-ignored truce in western Sudan.
UN spokeswoman Radhia Achouri said only two aid agencies were working in Muhajiriya and they were unable to cope with the strain of the new arrivals fleeing fighting in nearby Gereida in south Darfur.
Ms Achouri said it was not clear which rebel groups were involved in the fighting, and described the humanitarian situation as 'alarming'.
The towns of Muhajiriya and Gereida are controlled by the Minni Arcua Minnawi SLA faction, the only one of three negotiating rebel factions to sign a May peace deal with the government.
Khartoum has refused a UN takeover of the cash-strapped and struggling AU force in Darfur.
The AU has extended its mandate until the end of the year but has indicated it is unwilling and unable to continue in Darfur indefinitely.
It is thought the Sudanese govt fears UN troops will be used to arrest officials likely to be indicted by the International Criminal Court investigating alleged war crimes in Darfur.
Tens of thousands have been killed and 2.5 million forced to flee their homes during almost four years of rebellion in Darfur.
Rape and murder have been widespread and many of the victims have been from the non-Arab tribes who initiated the revolt.