The United Nations' top aid official on Wednesday said a three-hour truce announced by Russia to deliver aid to Aleppo would not be enough to meet the needs of civilians in the war-battered Syrian city.
"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.
O'Brien said he had not been fully briefed on the Russian proposal, but that there were complicated logistics to address, such as ensuring that truck drivers have enough time to safely make the trip to the city and back.
"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.
"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."
Fighting between government forces and rebels in Aleppo has intensified over the past month, with both sides sending in reinforcements for an all-out battle that could mark a turning point in the five-year war.
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 on Tuesday renewed his call for a 48-hour pause during his meeting with the UN Security Council