The United States was scrambling tonight to put the best spin on intelligence it has so far that purportedly pins last week’s chemical attacks in the outskirts of Damascus on the regime of President al-Assad amid concern that it may not be enough to convince sceptics in Congress and an uneasy American public.

Members of President Barack Obama’s foreign policy team, including Secretary of State John Kerry and Defence Secretary Chuck Hagel, were set to lay out the justification for a military strike on Syria to congressional leaders by video-conference this evening. A redacted version of that presentation was likely then to be made public.

President Obama continues to assert that the regime ordered the use of chemical weapons killing hundreds of people on 21 August and that such an act cannot go by without punishment.

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“We have concluded that the Syrian government in fact carried these out,” he told PBS on Wednesday night. Yet he faces headwinds selling that case with sources indicating that the intelligence dossier as it stands now falls far short of definitively linking the attacks to Assad.

One source who is familiar with it told the Associated Press that it is “no slam dunk” that Assad or his top lieutenants ordered the attack, a choice of words that has clear undertones. It was the then CIA director, George Tenet, who in 2002 boasted that it was a “slam dunk” that Saddam Hussein had weapons of mass destruction (WMDs) in Iraq. Those weapons, of course, were never found.

Mr Obama faces other hurdles, including pulling together allies to take part. Even the participation of Britain looked less certain. An attempt to get a resolution through the UN Security Council was thwarted by Russia, which tonight called for an emergency meeting of the Council to discuss the crisis further. On top of that, some members of Congress were demanding that Mr Obama seek approval from them before acting.

Even the White House seemed keen to lower expectations ahead of the release of the intelligence dossier, which may be the most important such presentation since Colin Powell, then Secretary of State, did his slide show allegedly removing doubts about WMDs in Iraq. There will be “no smoking gun” linking the chemical attack to President Assad, one White House official conceded.

Meanwhile those sources familiar with the dossier say it is filled with caveats and holes. It reportedly does not tie the attacks indisputably either to Assad or even to members of his inner circle. And while there is an intercept of a phone conversation about the incident apparently between Syrian Army commanders, there is no saying how senior they are or how close to the president.

Thus the US administration cannot be certain the gas wasn’t released by rogue elements in the army or, more worryingly, by al-Qa’ida fighters trying to draw the West into the conflict.

The CIA and the Pentagon have relied on the spy agencies of Jordan, Saudi Arabia and Israel for much of the material. Seemingly it will also show that they have largely lost track of where Assad’s chemical weapons are stored, raising the possibly that cruise missile strikes could destroy facilities containing those weapons, possibly triggering an even greater civilian disaster.

Among those demanding more direct congressional involvement was Barbara Lee, a Democrat congresswoman from California. “While the use of chemical weapons is deeply troubling and unacceptable, I believe there is no military solution to the complex Syrian crisis,” she said. “Congress needs to have a full debate before the United States commits to any military force in Syria – or elsewhere.”