It is too dangerous for aid to be taken into the western Sudanese region of Darfur on Monday by road as planned.
Thousands of displaced people are in need of relief supplies
Last week, the government opened humanitarian corridors for aid agencies to access Darfur where rebels have been battling the army and Arab militias.
But the United Nations' Dr Mukesh Kapila told the BBC that aid convoys were still being attacked by militias.
Dr Kapila called for international support for an airlift for the 1m people estimated to be in need.
Most are internally displaced but over 100,000 have fled into neighbouring Chad.
Aid agencies estimate that the fighting has cut off some three million people for several months.
The conflict in western Sudan began last year and is an entirely separate one from the 20-year battle involving the southern-based Sudan People's Liberation Army.
However, there are fears that the fighting risks overshadowing a looming peace deal with rebels in the south of the country.
'Rebels in control'
Dr Kapila said any convoys were also at risk from mines laid along the roads.
He said small aircraft were now taking some food and medicines into the more remote communities, but that this was no substitute for access by road.
"The road options are by far the most important but that clearly depends on security and that's what we are pursuing with the government authorities," he said.
Dr Kapila has called on the international community to help with large scale transport aircraft to airlift urgently needed food and blankets for the refugees in Darfur.
The Sudanese government says it is in full control of Darfur but rebels from the Justice and Equality Movement (Jem) and Sudan Liberation Army (SLA) insist they control much of the countryside.
A statement issued by the government last week said humanitarian organisations can now access the towns of al-Fashir, Kutumu, Amiro, Karnoi and Tine.
Other areas said to have been opened by the government are Jinnenah, Zalingei, Morni and Kolbus.