Report released by White House claims at least 1,429 people were killed with an unspecified nerve agent in Damascus suburb

With "high confidence," US intelligence assesses that forces loyal and responsive to Syrian dictator Bashar al-Assad killed at least 1,429 people with an unspecified nerve agent on August 21.

The case, spelled out in an unclassified four-page dossier released by the Obama administration Friday afternoon, is widely seen as a prelude to a military strike on Syria in the coming days.

"This is evidence. These are facts," Secretary of state John Kerry announced on Friday.

The Obama administration's war planning suffered a major setback on Thursday when the UK parliament unexpectedly voted against contributing to the assault. Conspicuously, a UK intelligence dossier released Thursday, listed "at least 350 fatalities" in the Syrian chemical strike, rather than the 1,429, including "at least 426 children," cited in the American one.

Despite the British political defection, the US administration appears undeterred from launching an anticipated strike, although President Obama is said not to have made a final opinion on launching it.

If so, the US will have launched two military operations – likely differing significantly in scale – in a decade based largely on intelligence assessments, rather than confirmed fact, about illegal weapons in a Middle Eastern country.

Unlike with the 2003 case against Iraq's Saddam Hussein, the existence of Assad's chemical stockpiles is not in doubt. The incident in question, a chemical attack outside Damascus on August 21, was heavily covered over social media at the time and since – something referenced in the assessment.

But the intelligence dossier also references directly collected surveillance on Syrian officials, as first reported by Foreign Policy magazine, as well as human sources, geospatial data and even public social media information.

"We have a body of information, including past Syrian practice, that leads us to conclude that regime officials were witting of and directed the attack on August 21," the dossier reads, laying out the closest thing the document presents to a smoking gun.

"We intercepted communications involving a senior official intimately familiar with the offensive who confirmed that chemical weapons were used by the regime on August 21 and was concerned with the UN inspectors obtaining evidence. On the afternoon of August 21, we have intelligence that Syrian chemical weapons personnel were directed to cease operations."

"I don't think there's any doubt to the world that a chemical weapons attack took place given the thousands of sources," a senior administration official told reporters on a Friday conference call, although the official insisted on not being named.

But not included in the variety of intelligence sources underlying the assessment are actual physiological samples from the attack – which the US seems not to possess.

"We have physiological samples from the previous assessment" that Assad's forces used chemical weapons in an earlier incident, an official said, "but given, again, how shortly ago this attack took place, we have not included physiological samples in this assessment. That would have to be something that would have a longer time lag in terms of collecting that type of information."

According to the dossier and to administration explanations of it, US intelligence collected "in the three days prior to the attack," data indicating "preparations" for the assault. Administration officials said "some built-in delay" of some of the threat data prevented the US from preventing the deadly attack – something Army General Martin Dempsey, the chairman of the Joint Chiefs of Staff, warned in January was "almost unachievable."

Officials said a year's worth of both public US statements and diplomatic demarches, including some delivered directly to the Assad regime, were intended to warn the Syrian government not to use chemical weapons.

But the August 21 attack, according to a map released by the Obama administration, took place in 12 neighborhoods near the capital either controlled by anti-Assad rebels or in contested areas. A senior administration official chalked up Assad loyalists' motivation as "frustration" with their inability to decisively reclaim the territory, and to "free up resources held down" by fighting in the neighborhoods. Rocket barrages preceded the chemical attacks by up to approximately 90 minutes on August 21.

"Syrian chemical weapons personnel were operating in the Damascus suburb of Adra from Sunday, August 18 until early in the morning on Wednesday, August 21 near an area that the regime uses to mix chemical weapons, including sarin," the document reads – although it does not specify that sarin was the nerve agent employed in the attack.

Michael Kuhlman, the chief scientist for national security for the Battelle corporation, said that the forensic traces of different chemical weapons, including different nerve agents, were distinct – if not necessarily easy to distinguish. VX is distinguishable from sarin and soman, for instance; but sarin and soman might not be easy to distinguish from each other.

"The agents themselves are quite readily distinguishable, even at trace levels," Kuhlman said in an email.

"If, however, one is relying upon detection of degradation products in the environment, the differentiation is a bit more complicated. The degradation products of sarin and soman [nerve agents], for example are readily distinguished from those of [the nerve agent] VX, but distinguishing between those two (sarin and soman) based solely on degradation products is more difficult. These analyses would rely on mass spectroscopic analytical methods."

He continued: "The nerve agents, sarin and soman, for example, differ only in the alcohol group that is attached to the organophosphorus portion of the molecule – thus, their degradation products are similar. VX has a quite different structure, and so has quite different degradation pathways it follows. Additionally, the volatility of VX is extremely low, so that it will be more likely to remain detectable in a sample."

Paul Pillar, a former senior CIA analyst whose analysis of a difficult postwar Iraq occupation was vindicated, said that the confident unclassified intelligence assessment was what he expected after senior administration officials, including President Obama, issued categorical statements that the chemical attack took place.

"I think it is a fair reaction of the public to say that it indeed looks highly likely that Syrian government forces used a nerve agent in an attack last week," Pillar said.

"The paper does not, however, dispose of the question of whether this was an attack ordered from the top or taken on the initiative of lower levels. It certainly does not dispose of the policy issue of what, if anything, the U.S. should do in response to this civil war."

A senior administration official said that Obama had not disposed of that issue, either.

"The president has not made a final decision on a course of action," the official said.

"We feel like our case is strong, our case is clear, the Assad regime is responsible for this mass casualty chemical weapons attack."

Asked why Obama had yet to make a final determination on how to enforce the "red line" he has said chemical weapons use represents, the official replied: "It's important for the president to be prudent and careful in his decisionmaking."

A decision appears to be imminent. The official said Obama "has received options from the military and his broader national security team about potential courses of action."

But any such military response will be, the official said, "limited and tailored and focused on the use of chemical weapons."