Russian forces are due to suspend military action in the northern Syrian city of Aleppo shortly to allow in desperately needed humanitarian aid.
It is not clear if Syrian rebel groups will also cease fire during the daily three-hour window.
But UN officials say that is too short to take in enough to help the large numbers of people in need.
Intense fighting has been continuing in Aleppo between rebels and Russian-backed Syrian government forces.
There are also reports of a toxic gas attack on a rebel-held area. Medical staff say four people died and many other were injured. The gas is thought to have been chlorine dropped in a barrel bomb.
Earlier the last doctors in the rebel-held east of the city appealed to US President Barack Obama to come to the aid of the 250,000 civilians there.
All Russian military action, air and artillery strikes would be halted between 10:00 (07:00 GMT) and 13:00, a defence ministry official told a briefing in Moscow on Wednesday.
But UN Emergency Relief Co-ordinator Stephen O'Brien that to meet the needs "you need two (road) lanes and you need to have about 48 hours to get sufficient trucks in."
"United Nations agencies and our partners remain ready to assist the civilian population across Aleppo. We have supplies ready to roll - food rations, hospital supplies, ambulances, fuel for generators, water supplies and more.
"We will continue to use all available routes and mechanisms to do this, including cross-line and cross-border operations from Turkey. We can deliver these within 24-48 hours - if we have safe access," Mr O'Brien said.
Fighting has escalated in Aleppo in recent days, with rebels severing the government's main route to the west of the city.
The offensive sought to break a siege by pro-government forces, who encircled the east in July with the support of Russian aircraft.
The remaining Aleppo doctors say in their letter that in the five years since the uprising against President Bashar al-Assad began, they have "borne witness as countless patients, friends and colleagues suffered violent, tormented deaths".
"The world has stood by and remarked how 'complicated' Syria is, while doing little to protect us. Recent offers of evacuation from the regime and Russia have sounded like thinly veiled threats to residents - flee now or face what fate?"
They say that in the past month there have been 42 attacks on medical facilities in Syria, 15 of them on hospitals where they work.
On Monday, the UN said countless civilians had been killed or injured over the past few weeks in the city, and that the targeting of hospitals and clinics had continued unabated.
Moreover, attacks on civilian infrastructure had left more than two million people without electricity or access to the public water network for several days, it added.
Credit: The BBC