Aid could reach a besieged Syrian village as early as Sunday morning, the World Food Programme said.
A convoy will carry one month's worth of food and medicine for the 40,000 people trapped in Madaya, a rebel-held town near the Lebanese border.
People there are reported to have been starving to death, with people eating pets and grass to survive.
A similar operation for two government-held villages in the north - Kefraya and Foah - is due on Monday.
Blockades have been a feature of Syria's civil war but the plight of Madaya has drawn international attention, partly due to images emerging of severely malnourished residents.
Up to 4.5 million people in Syria live in hard-to-reach areas, including nearly 400,000 people in 15 besieged locations who do not have access to life-saving aid.
Madaya has been besieged since early July by government forces and their allies in Lebanon's Shia Islamist Hezbollah movement.
The WFP had earlier said that Monday was the earliest likely date for aid to be delivered. Convoys will have to cross front lines, meaning delays remain a possibility.
The situation in Foah and Kefraya, under siege from rebels, is also reported to be worsening, with an estimated 30,000 people trapped.
Meanwhile the UN's mediator in the Syrian conflict, Staffan de Mistura, is in Damascus trying to lay the groundwork for peace talks planned later this month.
Opposition leaders have made the lifting of sieges a condition of taking part in the talks.
What's happening in Syria?
More than 250,000 Syrians have lost their lives in almost five years of conflict, which began with anti-government protests before escalating into a brutal civil war. More than 11 million others have been forced from their homes as forces loyal to President Bashar al-Assad and rebels opposed to his rule battle each other - as well as jihadist militants from Islamic State.
Why are civilians under siege?
All parties to the conflict are using siege warfare, encircling populated areas, preventing civilians from leaving and blocking humanitarian access in an attempt to force opponents to surrender. Shortages of food, water, medicine, electricity and fuel have led to malnutrition and deaths among vulnerable groups.
Where are the sieges?
Government forces are besieging various locations in the eastern Ghouta area, outside Damascus, as well as the capital's western suburb of Darayya and the nearby mountain towns of Zabadani and Madaya. Rebel forces have encircled the villages of Foah and Kefraya in the northern province of Idlib, while IS militants are besieging government-held areas in the eastern city of Deir al-Zour.