"To meet that capacity of need, you need two lanes and you need to have about 48 hours to get sufficient trucks in," Stephen O'Brien told reporters.
Russia's military announced a three-hour daily halt of fighting starting on Thursday to allow humanitarian convoys to reach Aleppo, which has been engulfed in heavy fighting.
The United Nations has called for 48-hour weekly pauses for the aid deliveries.
"When we are offered three hours, you have to ask what can be achieved in that three hours," said O'Brien, the UN under-secretary general for humanitarian affairs.
More From This Section
"Is it to meet the need or would it only just meet a very small part of the need?
"Clearly from our point of view, we are simply there to meet all the need and we need to have sufficient capacity to do that."
Up to two million people in Aleppo have gone without running water for the past four days, UN agencies said, raising the risks of disease in a city already devastated by years of fighting.
O'Brien yesterday renewed his call for a 48-hour pause during his meeting with the UN Security Council.