WASHINGTON—The U.S. and Israel are closely monitoring Syria's suspected cache of weapons of mass destruction, fearing that terror groups could take advantage of the revolt against President Bashar al-Assad to obtain blistering agents, nerve gas and long-range missiles, according to officials from both countries.

U.S intelligence services believe Syria's nonconventional weapons programs include significant stockpiles of mustard gas, VX and Sarin gas and the missile and artillery systems to deliver them.

United...