No proof that Assad used it. Only PNAC has demonstrated agenda to implicate Assad, well known to students of US neoconservatism, but widely popularized by video testimony of US four star general Wesley Clark, first introduced to the neocon strategy in a 1991 Pentagon briefing by then Undersecretary of Defense Paul Wolfowitz, U. of Chicago PhD degree disciple of "father of neoconservatism" Prof. Leo Strauss. In 1998 Strauss' neocon disciples founded the "Project for a New American Century" (PNAC), to harness the US into the role of "Global Policeman", wherein according to the 1999 PNAC manifesto (Wikipedia PNAC), a "catalyzing new Pearl Harbor event" was prerequisite (to arrive on 9/11/2001) In 2000, Clark received a second Pentagon briefing detailing PNAC plans for the Pentagon to attack 7 countries in 5 yerars, including Iraq, Libya, Syria, and Iran. Scores of internet articles, documentary films, and video news reports both document and analyze Syrian jihadist use of poison gas on multiple occasions, and their false flag nature. This includes multiple leaks and hacker supplied intercepted email intercepts documenting the the false flag nature of these attacks framing Assad, as well as the Saudi role in supplying the poison gas itself to their jihadist proxy army in Syria, for the express purpose of drawing America into the battle to topple Assad.
Russia vetoes UN resolution on MH17 special tribunal (Reuters)
from the article: Europe urges U.S. to await UN report on Syria before any strike