U.N. humanitarian leaders are pleading with the warring parties in Syria to stop bombing schools, to allow aid workers access to the sick and to overcome a "political failure" to end an intractable conflict.

Leaders from several U.N. agencies including the World Health Organization and UNICEF on Tuesday described a worsening situation for health care and children as the U.N. special envoy for Syria prepares to host peace talks in Geneva among government and opposition leaders, for the first time in nearly two years.

U.N. humanitarian coordinator for Syria Yacoub El Hillo said "the suffering is immense" and said the message for envoys attending is: "Enough is enough."

A day earlier, U.N. special envoy for Syria Staffan de Mistura announced plans to hold peace talks involving multiple parties starting Friday.