The U.N.'s special envoy for Syria is calling for the U.S., Russia and the world community to support possible airdrops of food and other critically needed supplies to besieged areas of Syria.

Staffan de Mistura said Friday that Syria's government is blocking access to 14 locations, the opposition to President Bashar Assad to two more sites and the Islamic State extremist group to one.

He said delivering humanitarian aid is a priority, and since it now can't be done by land, "the option of airdrops or airlift need to be pursued."

De Mistura said the next step will be for the U.N. Security Council to consider the option. He called for active American and Russian backing, saying "there is a limit to what the U.N. can do" on its own.