John Kerry: "We know rockets came only from regime-controlled areas"
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US Secretary of State John Kerry has accused Syrian government forces of killing 1,429 people in a chemical weapons attack in Damascus last week.
Mr Kerry said the dead included 426 children, and described the attack as an "inconceivable horror".
President Barack Obama later said the US was considering a "limited narrow act" in response.
Syria has dismissed Mr Kerry's statement as "full of lies", insisting the rebels carried out the attack.
State-run news agency Sana said Mr Kerry, who cited a US intelligence assessment, was using "material based on old stories which were published by terrorists over a week ago".
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Jonathan MarcusBBC diplomatic correspondent
Mr Kerry's speech was a clear and powerful statement of the rationale for military action against Syria. The focus was placed entirely upon deterring the Syrian authorities from ever using chemical weapons again. This was neither an intervention in the civil war nor an attempt to topple the Assad regime.
Mr Kerry sought to convince a US public that is tired of war that it was Washington's responsibility to act. The message was that there would be no need to wait for the UN inspectors' report; he insisted the report would tell Americans nothing they didn't know already.
The stage is set for action. While no final decision has yet been taken to strike, it may only be a matter of days.
The US says its assessment is backed by accounts from medical personnel, witnesses, journalists, videos and thousands of social media reports.
UN chemical weapons inspectors are investigating the alleged poison-gas attacks and will present preliminary findings to the UN after they leave Damascus on Saturday.
But Mr Kerry said the US already had the facts, and nothing that the UN weapons inspectors found could tell the world anything new.
He highlighted evidence in the assessment that regime forces had spent three days in eastern Damascus preparing for the attack.
"We know rockets came only from regime-controlled areas and landed only in opposition-held areas," he said.
"All of these things we know, the American intelligence community has high confidence."
Mr Kerry called Mr Assad "a thug and a murderer" but said any response by the US would be carefully measured and would not involve a protracted campaign like Iraq or Afghanistan.