Factbox - What is the chemical weapon sarin?

(Reuters) - Western forces could attack Syria within days, the United States and its allies have told rebels fighting President Bashar al-Assad, in order to punish the Syrian government for a deadly poison gas attack that killed hundreds of civilians.

The Syrian government and the opposition have accused each other of using chemical weapons, and both have denied doing so. Activists say hundreds of civilians were killed in the rebel-held suburbs of Damascus on Wednesday, possibly by the nerve gas sarin or something similar.