The Syrian government has approved humanitarian access to seven besieged areas, the UN has said.
Convoys were being prepared to depart “as soon as possible”, said spokesman Farhan Haq.
Among the areas due to receive aid is Madaya, where people have been dying of starvation.
World powers last week agreed to seek a nationwide “cessation of hostilities” and to accelerate and expand aid deliveries.
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After talks in Damascus on Tuesday, the UN special envoy to Syria, Staffan de Mistura, said the Syrian government had a duty to allow access to whoever needed it.
This obligation, he said, would be put to the test on Wednesday.
The seven areas are those deemed by the 17-member International Syria Support Group to be most in need of relief.
“Humanitarian agencies and partners are preparing convoys for these areas to depart as soon as possible in the coming days,” said Mr Haq.
Almost half a million people live in besieged areas, according to the UN.
The truce, which does not apply to jihadist groups, is due to come into force later this week.
But Syrian President Bashar al-Assad has cast doubts on the cessation, warning it would be “difficult” to implement and would not mean all parties would stop using weapons.
His forces, backed by Russian air power, have been advancing in the north and threaten to surround the key city of Aleppo.
Where the aid is going
The seven areas named by the UN are:
- Deir el-Zour, a city in the east under siege from so-called Islamic State
- Foah and Kefraya, in northern Idlib province, besieged by rebels
- Madaya, Muadhamiya, Kafr Batna and Zabadani, all in the Damascus area under siege from government forces
Earlier Russia said it “categorically rejects” accusations of war crimes over the bombing of hospitals in Syria.
Turkey has blamed Russia for a series of rocket attacks on several hospitals and schools that killed up to 50 people.
But Kremlin spokesman Dmitry Peskov told the BBC that the only proof Russia would accept from the ground “comes from the Syrian authorities”.
Save the Children has said that seven healthcare facilities were hit by strikes on Monday – more than previously reported.