Washington’s Blog 
Sept 2, 2013
Antiwar argues :
The only reason the whole allegation [that the Syrian government used chemical weapons] hasn’t been dismissed out of hand is the administration’s repeated claims that they have better, secret intelligence  that they’re just not showing anyone, or occasionally are letting already pro-war Congressmen get a glimpse of.
But – as the Weekly Standard reports  – some Congressmen who have gotten a classified briefing aren’t impressed:
“I have just attended a classified Congressional briefing on Syria that quite franklyraised more questions than it answered. I found the evidence presented by Administration officials to be circumstantial” …. liberal Democrat Tom Harkin says in a statement released after today’s classified Capitol Hill briefing.
The Guardian reports :
As members of Congress emerged from the briefing, it was clear that the Obama administration could not be sure of the outcome of the president’s high-risk strategy. In particular, Obama could not count on his own party to deliver the votes. “I don’t know if every member of Congress is there yet,” said Representative Janice Hahn, a California Democrat who said she would vote no on authorising a military strike. “The room was sceptical,” said Jim Himes, a Connecticut Democrat.
The U.S. government’s main “classified” claims is that it intercepted communications from the Syrian government both before and after the chemical weapons attack which implicate at least some people  in Assad’s government. For example, the Washington Post notes :
Unknown to Syrian officials, U.S. spy agencies recorded each step in the alleged chemical attack, from the extensive preparations to the launching of rockets to the after-action assessments by Syrian officials. Those records and intercepts would become the core of the Obama administration’s evidentiary case linking the Syrian government to what one official called an “indiscriminate, inconceivable horror” — the use of outlawed toxins to kill nearly 1,500 civilians, including at least 426 children.
Who would have intercepted those communications?
You guessed it … likely the National Security Agency, the American agency tasked with intercepting foreign communications.
Former British ambassador Craig Murray argues that the U.S. claim is hogwash :
It is therefore very strange, to say the least, that John Kerry claims to have access to communications intercepts of Syrian military and officials organising chemical weapons attacks, which intercepts were not available to the British Joint Intelligence Committee.
On one level the explanation is simple. The intercept evidence was provided to the USA by Mossad, according to my own well placed source in the Washington intelligence community. Intelligence provided by a third party is not automatically shared with the UK, and indeed Israel specifies it should not be.
But the inescapable question is this. Mossad have nothing comparable to the Troodos operation [Britain’s top spying facility]. The reported content of the conversations fits exactly with key tasking for Troodos, and would have tripped all the triggers. How can Troodos have missed this if Mossad got it? The only remote possibility is that all the conversations went on a purely landline route, on which Mossad have a physical wire tap, but that is very unlikely in a number of ways – not least nowadays the purely landline route.
Israel has now provided “intelligence” to the United States designed to allow the United States to join in with Israel’s bombing and missile campaign.
The answer to the Troodos Conundrum is simple. Troodos did not pick up the intercepts because they do not exist. Mossad fabricated them. John Kerry’s “evidence” is the shabbiest of tricks.