The United Nations says the number of besieged areas in Syria's conflict has risen to 18, up from 15 earlier this month.

As the U.N. hopes to get Syrian parties to begin peace talks on Friday, the U.N. humanitarian chief and the head of the World Food Program on Wednesday called upon Syria's government to allow sustained access to as many as half a million people in besieged areas.

Officials said the rare convoys that reached a few besieged communities earlier this month are not enough and that the aid delivered will soon run out.

U.N. humanitarian chief Stephen O'Brien calls the idea of airdropping aid "risky" and insufficient, but all options are still being discussed.