Reports suggest that more than 100,000 people in Syria have been detained, abducted or gone missing during the eight-year conflict, with the government mainly responsible, the U.N. political chief said Wednesday.

The assault has raised fears of a humanitarian crisis as Syrians displaced by the fighting seek shelter at the Turkish border.

The U.N. deputy humanitarian chief warned Tuesday that further military operations in northwest Syria's Idlib province will overwhelm aid efforts, stressing that an estimated 3 million people are caught up in crossfire in the last rebel stronghold.