http://web.archive.org/web/20130129213824/http://www.dailymail.co.uk/news/article-2270219/U-S-planned-launch-chemical-weapon-attack-Syria-blame-Assad.html
http://www.dailymail.co.uk/news/article-2320223/UN-accuses-Syrian-rebels-carrying-sarin-gas-attacks-blamed-Assads-troops.html
This is just a fabricated story with no solid evidence to back it up. There are already thousands of conspiracy theory against the movements of the United States of America and due to this we cannot really identify the factual from the hoax. That is why I dont believe this article not until all solid and concrete, real evidence will be presented to authenticate that claim.
The Americans have done that before and I have no doubt that they will do it again in the future. Remember claims by the CIA that Iraq under Saddam Hussein was amassing weapons of mass destruction just before the NATO invasion of that country? What happened to those weapons? They were never found.