PARIS — Seeking to move beyond Britain’s blindsiding rejection of military intervention in the Syrian conflict, the Obama administration received strong support from France on Friday and released a detailed intelligence summary to buttress its contention that the Syrian government used chemical munitions in an Aug. 21 attack, asserting for the first time that it had killed 1,429 people, nearly a third of them children.

The intelligence summary, cited by Secretary of State John Kerry in a televised briefing at the State Department, contained by far the most specific contentions the administration has publicly made in arguing that the Syrian government had crossed a threshhold of intolerable behavior in its effort to defeat insurgents in the civil war, justifying an international military response.

The administration had previously not asserted such a precise death toll in the Aug. 21 attack, which rights activists and medical aid workers say left hundreds asphyxiated in what appeared to be the most egregious mass killing in the conflict, now in its third year.

A summary of the intelligence assessment said its conclusions were based on “human, signals and geospatial intelligence as well as a significant body of open-source reporting.”