The US, UK and France have urged the UN to begin air drops of humanitarian aid to besieged areas in Syria.
They said the Syrian government had failed to respect a 1 June deadline for widespread aid distribution agreed by world and regional powers.
About 4,000 people in the besieged Darayya suburb of the capital Damascus have been without food aid since 2012.
A convoy reached the area on Wednesday but carried only a small amount of medicines and other non-food items.
The UN Security Council will meet on Friday to discuss the air drops.
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.
US state department spokesman John Kirby said hundreds of thousands of Syrians needed “sustained and regular” access to aid.
He said the UN food agency, the World Food Programme (WFP), had briefed the US on how the air drops could be carried out.
Meanwhile 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”.
“While air drops are complex, costly and risky, they are now the last resort to relieve human suffering across many besieged areas,” he said.
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.
Russia said the arrival of Wednesday’s aid convoys was a positive step.
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.
The ISSG, co-chaired by the US and Russia, comprises 17 world and regional powers, as well as the Arab League, the European Union and the United Nations.
In April, the UN said at least 4,000 people were besieged in Darayya by Syrian government forces, which have surrounded the suburb for four years.
The Darayya convoy, the first since November 2012, contained vaccines, baby milk, medicine and nutritional goods.
Separately, the rebel-held town of Muadhamiya, north-west of Darayya, received deliveries of food parcels and wheat flour on Wednesday, a month since aid convoys last visited.
Darayya’s electricity supply was cut off more than three years ago.
UN emergency relief co-ordinator Stephen O’Brien said in April that the Syrian government had ignored “countless” requests for aid to be allowed in.
An aid convoy was blocked from entering the town last month, despite all involved parties agreeing aid could be delivered.
The latest delivery was made by teams from the UN, the International Committee of the Red Cross and the Syrian Red Crescent.
Russia’s defence ministry said on Wednesday it had agreed a pause with the Syrian authorities for 48 hours to allow for the distribution of humanitarian aid in Darayya.
It borders a military airport used by Russian planes in support of the government of President Bashar al-Assad.
It was one of the first towns to report demonstrations against the Assad regime, and has been under siege since late 2012. The UK-based Syrian Observatory for Human Rights reported clashes this week between rebel groups and government fighters on the outskirts of Darayya.
The UN believes there are more than 4.6m people living in hard-to-reach areas in Syria, including nearly 600,000 in besieged areas.