Almost all countries in the world have joined the Chemical Weapons Convention. Currently 188 of the 195 states recognized by the United Nations are party to the CWC. Of the seven states that are not, two have signed but not yet ratified the treaty (Burma and Israel) and five states have not signed the treaty (Angola, North Korea, Egypt, Somalia, and Syria).
U.S. to defend Syrian rebels with airpower, including from Assad (Reuters)
from the article: Assad forces mull use of chemical weapons in Homs, opposition says