Air drops of vital aid to besieged areas of Syria are "not imminent", a senior UN official has warned.
But Ramzi Essedine Ramzi, UN deputy special envoy for Syria, said the "process that will lead to air drops has already started".
He was speaking after the US, UK and France urged the UN to deliver by air.
They say the Syrian government has failed to respect a 1 June deadline for widespread aid distribution agreed by world and regional powers.
A convoy of aid reached besieged Darayya suburb of the capital Damascus on Wednesday, but it carried only a small amount of medicines and other non-food items. Darayya has been without food aid since 2012.
UN humanitarian coordinator Jan Egeland said a convoy of food for Darayya, planned for Friday, may be delayed but added there are "clear indications" it will go ahead within days.
The UN Security Council is expected to meet on Friday to discuss the question of air drops.
US state department spokesman John Kirby earlier said hundreds of thousands of Syrians needed "sustained and regular" access to aid.
British Foreign Secretary Philip Hammond described the limited access for aid allowed on the day of the deadline agreed by the International Syria Support group (ISSG) as "cynical".
He said air drops "are now the last resort to relieve human suffering across many besieged areas".
The US and UK called on Russia and Iran, which back the Syrian government, to use their influence to ensure the air drops could proceed safely.
But Mr Ramzi, speaking after a meeting of the UN's humanitarian task force for Syria, said: "Air drops... remain an option if land deliveries do not go through."
"I don't think it's imminent but I think the process that will lead to air drops has already started," he went on.
"It is not just the Russians who are concerned about security, it is an issue that has to be resolved in a way that allows this to go ahead."
The challenges of dropping aid by air
In February, the WFP carried out a 21-pallet air drop of aid to a government-held area of Deir al-Zour in eastern Syria, which ended in failure. Of the 21 pallets, 10 were unaccounted for, seven landed in no-man's land and four were damaged.
But it has since carried out 44 air drops over the city, delivering mainly rice, chickpeas and beans to meet the immediate needs of around 100,000 residents.
The WFP has identified some 592,700 people in 19 besieged areas of Syria that may need to receive aid by air.
But the operating conditions are challenging, the WFP says. Many besieged locations are in built-up, urban areas with no suitable space for a drop zone. High-altitude drops are not possible in those areas because of the risk of harming people on the ground.
The UN food agency says airdrops are always a last resort as access by land is more efficient.
Meanwhile, in the north of Syria, US warplanes have been trying to destroy a key access route to Turkey for militants from the so-called Islamic State (IS).
Overnight air strikes destroyed all the bridges linking the IS-controlled towns of Manbij to Jarablus, on the Turkish border, an IS-affiliated news agency confirmed.
The Kurdish-led Syrian Democratic Forces (SDF), which is leading the ground operation to recapture Manbij, says it is within 10km (six miles) of the town.