Helicopters could be used to deliver aid to besieged areas of Syria, the UN said, on the eve of an emergency meeting of the Security Council about the issue.
Humanitarian access in Syria has been a key sticking point in stalled UN-backed peace talks aimed at ending the five-year war that has killed at least 280,000 people and displaced millions.
United Nations spokesman Stephane Dujarric said helicopters would have to be used to carry aid to 15 of 19 besieged areas that include densely populated towns.
"In urban areas, airdrops are not feasible, so you are talking about the use of helicopters," he said.
"One can imagine the security challenges for that -- plus the security challenges of flying helicopters over the skies of Syria."
The comments came as a US-backed alliance of Kurds and Arabs pressed its advance in northern Syria towards the town of Manbij, held by the Islamic State jihadist group.
The US-led coalition also dropped ammunition to Syrian rebels fighting IS as they try to prevent the jihadists from entering the town of Marea, a monitoring group and US defence official said.
In Syria's second city, Aleppo, 23 civilians were killed by regime bombardments, the civil defence said, while a suicide bombing near a mosque killed at least two in the government coastal stronghold of Latakia.
The Security Council is due to meet on Friday to discuss the UN plan for airdrops.
Last month the United Nations said that if it did not see improvement on aid access to besieged areas by June 1, it would task its food agency to carry out drops.
- 'Too little, too late'-
A deputy to the UN special envoy to Syria Staffan de Mistura said "as long as the World Food Programme has not yet finalised its plans, I don't think there's something imminent".
Ramzi Ezzedine Ramzi said airdrops were "very complex" and would need Damascus' approval.
On Wednesday, land deliveries reached two towns besieged by government forces where civilians are facing severe food shortages.
A truce allowed a convoy to enter Daraya near Damascus for the first time since late 2012, while another entered nearby Moadamiyeh for the first time since March.
But the opposition said only medical supplies were in the Daraya delivery and charity Save the Children said it was "shocking and completely unacceptable" that it excluded desperately needed food.
"It's too little, too late," said Matthew Rycroft, the British ambassador to the UN, referring to Wednesday's deliveries.
An estimated 8,000 people live in Daraya, one of the first towns in Syria to erupt in anti-government demonstrations in 2012 and one of the first to come under a strict regime siege the same year.
"Daraya needs everything," said local activist Shadi Matar.
"Some items come in through smuggling because some people risk their lives to slip out of town."
The UN says a total of 592,000 people live under siege in Syria -- most surrounded by government forces -- and another four million in hard-to-reach areas.
- Anti-IS offensive -
Ambassador Francois Delattre of France, which currently chairs the UN Security Council, called for airdrops to all areas in need and blamed the regime for blocking access to besieged villages and towns.
Moscow, a staunch ally of the Syrian regime, said the Daraya truce would be observed until 00:01 am on Friday (2101 GMT Thursday) to allow aid deliveries.
But Russia's ambassador Vitaly Churkin suggested plans to airdrop humanitarian relief could be put on hold.
"I think we need to continue to pursue with land deliveries," he said.
In north Syria, fighters from the Kurdish-Arab alliance were 10 kilometres (six miles) outside the IS-held town of Manbij, a monitor said.
The Syrian Democratic Forces alliance has seized some 20 villages outside the town, said the Syrian Observatory for Human Rights.
The US-backed SDF last week launched an offensive north of the jihadists' Syrian stronghold of Raqa city.
This week the alliance opened a new front towards Manbij, which lies on a key supply route between Raqa and the IS-held town of Jarabulus on the Turkish border.
Syria's main opposition group on Wednesday called on the UN to implement a truce in all of the country except areas under IS control during the Muslim holy month of Ramadan, which begins next week.
-Beirut (AFP) -