The United Nations and partner aid organisations plan to deliver life-saving aid to 154,000 Syrians in besieged areas in the next five days.
Pending approval from parties to the conflict, the UN is ready to deliver aid to about 1.7 million people in hard-to-reach areas in the first quarter of 2016, resident co-ordinator in Damascus Yacoub El Hillo said on Sunday (local time).
The UN estimates almost 500,000 people in Syria are living under siege, out of 4.6 million who are hard to reach with aid, but it hopes a cessation of hostilities that began on Friday night will bring an end to the 15 sieges.
"It is the best opportunity that the Syrian people have had over the last five years for lasting peace and stability," El Hillo said.
"But we all know that without a meaningful political process and a political solution, both cessation of hostilities and entry of humanitarian assistance will not be enough to end the crisis in Syria."
The UN hopes to deliver aid to Moadamiya on Monday, Zabadani, Kufreya, Foua and Madaya on Wednesday, and Kafr Batna on Friday.
But the biggest single siege, of about 200,000 people in Deir al-Zor, is not affected by the cessation of hostilities because the besieging Islamic State forces are excluded from the agreement.
The UN attempted an air drop there last week but high winds meant all 21 tonnes of food went off target or went missing or their parachutes failed to open and they were destroyed.