Britain's Prime Minister David Cameron addresses the House of Commons in London on Aug. 29. (Reuters)

The British government laid out its case Thursday for possible military intervention over last week’s alleged chemical weapons attack in Syria, citing what it acknowledged was a “limited but growing” body of intelligence implicating the Syrian regime in the deaths of about 350 people.

But the brief summary released by Britain’s Joint Intelligence Committee offered few specifics and contained little to sway international skeptics, a task that British officials appear to have left to the Obama administration, which has promised to release its own intelligence assessment.

After the document was released, British Prime Minister David Cameron suffered a significant defeat in Parliament, raising serious doubts that Britain could participate in any action.

The British document referred mainly to video footage of the attack posted on the Internet and what officials called a pattern of past use of chemical weapons by Syrian President Bashar al-
Assad. It dismissed as improbable Syria’s claim that rebels had staged the attack.

The White House has sought to assure allies that it has evidence strongly linking the Assad regime to the alleged chemical attack, which would be the world’s deadliest use of such weapons in at least two decades.

The British government posted its intelligence assessment on Web sites Thursday, along with a two-page legal paper explaining its rationale for intervening militarily in Syria. The legal document blasted Syria for acts that it said constituted “a war crime and a crime against humanity.” It said Britain was compelled to respond for humanitarian reasons, saying force was needed to stop the Assad regime from further attacks.

“As an exceptional measure on the grounds of overwhelming humanitarian necessity, military intervention to strike specific targets with the aim of deterring and disrupting further such attacks would be necessary and proportionate, and therefore legally justifiable,” it said.

The intelligence document and an accompanying letter cited “a clear pattern of regime use” of deadly chemicals in the past, including 14 documented incidents since early 2012. The documents concluded that the alleged chemical attack on the Damascus suburbs on Aug. 21 is most likely to have involved the nerve agent sarin, a conclusion that it said was based on an analysis of symptoms of victims shown on widely distributed videos of the aftermath.

As for establishing the regime’s role in the alleged chemical attack, the summary said there was “some intelligence to suggest regime culpability,” but it did not elaborate other than to cite circumstantial factors, such as lack of other plausible culprits.

“None currently have the capability to conduct a CW attack on this scale,” the document said.

The accompanying letter, signed by Joint Intelligence Committee Chairman Jon Day, referred to additional “highly sensitive” intelligence that was not released and that showed that the motive for the alleged chemical attack was to clear rebel forces from strategic parts of Damascus.