A "concrete plan" was in place for the operation over the eastern city of Deir al-Zour, said Jan Egeland, who chairs a humanitarian taskforce.
Convoys of UN trucks on Wednesday began delivering much-needed aid to 80,000 people in five other besieged areas.
The UN says 200,000 residents remain besieged in parts of Deir al-Zour.
"It's a complicated operation and would be in many ways the first of its kind," Egeland said of the air drops.
The UN will work with the Syrian Arab Red Crescent and other local groups to drop aid by parachute for distribution, World Food Programme spokeswoman Bettina Luescher said.
She did not specify when the air drops would begin but said a single aircraft would be used initially.
The World Food Programme had previously ruled out humanitarian air drops in Syria due to the complexities of obtaining use of airspace, organising distribution on the ground, and finding suitable drop zones.
The UK government also said air drops were "high risk and should only be considered as a last resort when all other means have failed".
But Egeland said the strategy was the only way to feed people in Deir al-Zour.
"It is either air drops or nothing. Air drops are a desperate measure in desperate times," he told Reuters.
Speaking after a meeting of the 17-member International Syria Support Group (ISSG), he also said that UN aid was expected to reach all of Syria's 18 besieged areas within a week.
The UN believes more than 480,000 Syrians are living in besieged areas, with four million more people in so-called "hard-to-reach" areas.
All parties to the conflict are believed to have used siege warfare - where military forces surround an area and cut off essential supplies - in breach of international law.
On Wednesday, aid trucks reached rebel-held Muadhamiya and Madaya, near Damascus, and pro-government northern villages of Foah and Kefraya. Another town, Zabadani, was reached later.
The supplies, which included some medical supplies, are expected to last for about a month.
The deliveries were part of an agreement approved by the ISSG that world powers hope will lead to a "cessation of hostilities" by Friday.
The agreement does not apply to the fight against IS or al-Nusra Front, an al-Qaeda affiliate, and offensives by Syrian government forces and Kurdish militia fighters on rebel-held areas of the northern province of Aleppo have dimmed hopes of a truce.
A new report from humanitarian group Doctors Without Borders/Medecins Sans Frontieres (MSF) says 1.9 million people are under siege in Syria. It said data from hospitals and clinics it supports recorded 154,647 war-injured people and 7,009 war dead in 2015, 30 to 40 percent of whom were women and children.
The data, MSF said, showed "civilians and civilian areas continued to be devastated" by the conflict.
A strike on an MSF-supported hospital in Idlib province on Monday killed 25 people, the group says.
"Permanent members of the UN Security Council, four of whom are actively involved in the war in Syria, must answer for their failure to uphold their most basic responsibilities toward civilians," said MSF president Dr Joanne Liu.
The UN is planning to air drop food and other aid to a Syrian city besieged by the so-called Islamic State (IS), officials have said.