The United Nations on Thursday tempered expectations that it would imminently begin airlifting humanitarian aid to besieged areas of Syria, indicating it could not carry out the mission without the approval of the Syrian government.
"You need the consent of government," UN deputy Syria envoy Ramzy Ezzeldin Ramzy said at a press conference in Geneva, adding that he was not "aware" of any such permission. "I don't think there is something imminent," he cautioned.
He added that rebels and the government would have to comply to ensure staff safety during a "complex venture." Dropping supplies on urban areas would require helicopters, which could be exposed to attacks.
The International Syria Support Group (ISSG) - which includes the United States and Russia - last month pledged that if humanitarian aid was denied to besieged areas, the UN's World Food Programme (WFP) would be called on to carry out air drops starting June 1.
On Wednesday, the US, Britain and France called on the WFP to move ahead with planning to carry out the air drops, losing patience with the slow pace of deliveries on the ground.
Russia, the Syrian government's key ally, has been more tepid, pointing to some trucks entering besieged districts. UN officials admit aid deliveries in May were far below expectations.
Much blame is placed on the Syrian government, including removing vital supplies from trucks and stopping aid deliveries from reaching rebel-held enclaves.
Some 492,000 people are in 19 areas in Syria under siege, according to the UN. Of these, 16 areas are rebel-held while three are government-controlled.
"Air delivery remains an option," Ramzy said, stressing that the UN prefers to carry out humanitarian aid deliveries by land, as it is easier and much less expensive.
The UN Security Council is due to convene on the situation Friday.
Jan Egeland, a senior UN adviser on humanitarian affairs for Syria, said airlifts would soon begin to al-Hassakeh, in the far north, an area mostly under Kurdish control but with some government-held pockets.
The border from Iraq to the Kurdish areas of Syria has been closed of late, increasing the pressure on the local population. Turkey has also shut its border.
This week, some aid got into Daraya, a rebel-held suburb of Damascus with some 4,000 people, for the first time since 2012, after humanitarian trucks were blocked last month. However, it was only a partial delivery, and food supplies are still desperately needed.
"Daraya is very symbolically important for both sides," Egeland said, but added: "We believe we will be able to go there with food."
Meanwhile, US-backed forces have pledged to eliminate the Islamic State's presence in Minbij, in the north, as an offensive continued for a second day.
"All civilians are asked to cooperate to oust Islamic State from Minbij, and we call on all civilians living in areas near Islamic State bases or offices to stay away," a Kurdish official told dpa.
The US-backed Democratic Forces of Syria (DFS) - composed largely of the Kurdish People's Protection Units (YPG), but also including Arab and Turkmen groups - launched the offensive against the key Minbij pocket, the extremists' last territory on the Turkish border.
The mission, which appears spearheaded by Arab forces, aims to kick the extremists off the Turkish border - just 20 kilometres from Minbij - and supply routes, while also isolating Islamic State's de facto capital, al-Raqqa.
A separate, US-backed offensive against areas north of that city was launched last week.
Syrian rebels lost Minbij to Islamic State in early 2014 after seizing it from President Bashar al-Assad's forces in 2012.
Separately, Doctors Without Borders (MSF) said Turkey should open its borders to some 100,000 people fleeing fighting between rebel groups and the Islamic State in northern Aleppo province, while urging Europe to speed up the process of taking in refugees.
The people trapped in the shrinking Azaz corridor are facing "desperate conditions," MSF said. Rebel groups, including ones backed by Turkey, recently ceded territory to Islamic State, in a stunning defeat.