The UN's Syria envoy, Staffan de Mistura, on Thursday said if the country's regime continued blocking humanitarian aid to besieged towns supplies may be delivered by airdrops.
After the International Syria Support Group's Humanitarian Access Task Force meeting in Geneva, de Mistura told reporters: "If there is no substantial progress in humanitarian access to these areas by the first of June, there is an option of airdrops."
Any lack of improvement in humanitarian access or a cessation of hostilities would mean "the credibility of the next round of talks will be in question, therefore l am not going to give you a date yet," de Mistura added.
According to the UN, 13 out of 18 besieged areas have been reached so far although the regime blocks aid to the remaining locations.
The Syrian regime last Thursday refused entry to the town of Daraya, under siege since 2012.
UN airdrops delivered aid over the last months to Deir ez-Zor in eastern Syria, under siege from Daesh.
According to the UN, nearly one million people in besieged and hard-to-reach areas urgently need aid.
Syria has been locked in a vicious civil war since early 2011, when the Bashar Assad regime cracked down on pro-democracy protests with unexpected ferocity.
Since then, more than 250,000 people have been killed and more than 10 million displaced, according to the UN.