Original: As chemical weapons, they are classified as weapons of mass destruction by the United Nations according to UN Resolution 687 (passed in April 1991) and their production and stockpiling was outlawed by the Chemical Weapons Convention of 1993; the Chemical Weapons Convention officially took effect on April 29, 1997.
Simplified: Since they are chemical weapons, they are classified as weapons of mass destruction by the United Nations according to UN Resolution 687.