World powers on Tuesday called on the United Nations to carry out air drops over besieged areas in Syria as regime blockades in rebel-held areas were stopping all food and medical aid in defiance of UN Security Council resolutions.

"Starting on June 1, if the UN is denied humanitarian access to any of these designated areas, the International Syria Support calls on the World Food Programme to immediately carry out a programme for air bridges and air drops to all of those areas in need," US Secretary of State John Kerry said after conflict talks in Vienna.