The UN's top official in Sudan on Tuesday called for immediate access to southern Darfur where thousands of civilians have been affected by rebel-government fighting but relief teams have been denied access for security reasons.
The appeal by Ali Al-Za'tari came as rebels reported fresh fighting in the area on Tuesday.
Za'tari, citing government figures, said about 36,000 people have gathered around the bases of international peacekeepers in Muhagiriya and Labado, some 100 kilometres (60 miles) east of the South Darfur state capital Nyala.
"Reports received by the United Nations indicate an outbreak of diarrhoea among children requiring urgent and immediate assistance," Za'tari said, expressing "serious concern" for civilians in the area.
He said aid workers need access right away to assess the people's condition and provide relief supplies.
Za'tari urged the government and rebels of the Sudan Liberation Army's Minni Minnawi faction "to uphold their humanitarian responsibilities to protect all civilian populations."
He stressed that civilians wanting to leave the area must be allowed safe passage.
On Monday a UN official said "authorities deemed it too insecure" for aid agencies to enter Muhagiriya and Labado.
"This morning we resisted a government attack on Labado," the Minnawi faction's London representative, Nuraldim Mohammed Taha, told AFP on Tuesday.
The insurgents, who usually stage hit-and-run attacks, 10 days ago began an unusual occupation of the strategic area.
Sudan's Defence Minister Abdelrahim Mohammed Hussein vowed last week to move against them.
The army's spokesman could not be reached on Tuesday.
Minnawi and other ethnic rebels in Sudan's far-western Darfur region rose up against the Arab-dominated Khartoum government in 2003.
While the worst of the violence has long passed, rebel-government battles continue but instability has been complicated by inter-Arab fighting, kidnappings, carjackings and other crimes, many of them suspected to be the work of government-linked militia and paramilitary groups.