The UN’s first aid-drop over the Syrian city of Deir al-Zour, part of which is under siege by Islamic State militants, may have been off-target.
Wednesday’s operation “faced technical difficulties” and “adjustments” were needed, the World Food Programme said.
It added that it would try again when possible to deliver aid to the 200,000 civilians who have been trapped in a government-held area since March 2014.
Local activists said the aid was either damaged or fell wide of its mark.
The air-drop is part of a larger international effort to deliver desperately needed supplies of food and medicine to many of the 400,000 people living under siege.
UN aid chief Stephen O’Brien told the Security Council that initial reports from Syrian Arab Red Crescent (SARC) teams on the ground suggested that the first cargo of 21 tonnes of aid dropped over Deir al-Zour had landed in the target area as planned.
However, UN spokesman Stephane Dujarric later told reporters that the WFP was still trying to get information on where the aid ended up.
“The pallets were dropped,” he said. “They’re trying to reach local partners to ensure that the aid was received.”
“There may have been some difficulties in terms of the pallets,” he added.
A WFP statement said the operation “faced technical difficulties and we are debriefing crew and partners in Deir al-Zour to make necessary adjustments”.
“High-altitude drops are extremely challenging to carry out and take more than one trial to develop full accuracy,” it added.
An activist group, The Justice for Life Observatory in Deir al-Zour, said the cargo was significantly damaged and that the SARC had only been able to collect three of six containers because half had fallen into areas that are difficult to access, according to the Irin news agency.
Last month, UN humanitarian agencies warned that the civilians living in the besieged western side of Deir al-Zour, most of whom are women and children, were facing sharply deteriorating conditions.
Residents needed immediate and urgent humanitarian assistance, particularly food, nutrition and health supplies, and there were reports of severe cases of malnutrition and deaths due to starvation, they said.
While the Syrian government’s stocks continue to provide bread, there were very limited supplies as there has been limited humanitarian or commercial access to the eastern city.
Mr O’Brien also told the Security Council that since 17 February, the UN and its partners had reached 110,000 people in besieged areas, and had approval to reach a further 230,000 people, including through the air-drops in Deir al-Zour. They are still waiting for approval an additional 170,000, he said.
He also welcomed the plan announced by the US and Russia for a nationwide cessation of hostilities scheduled to come into effect at 2200 GMT on Friday.
Syria’s main opposition umbrella group said it was ready for a two-week truce to test the government’s commitment to the plan.
But the High Negotiations Committee objected to Russia being a guarantor, because of the air campaign it has conducted in support of President Bashar al-Assad since September.
The HNC also expressed concern that Moscow and Damascus would continue targeting rebels allied to the al-Nusra Front, a jihadist group that along with IS will be excluded from the cessation of hostilities.
The United Nations special envoy to Syria, Staffan de Mistura, meanwhile said he would be announcing the date for the next round of peace talks in Geneva on Friday.