The Syrian government has again denied it was behind the chemical attack that killed 1400 people on the outskirts of Damascus, and warned that any American military action against it would amount to support for al-Qaeda and its affiliates.
Syrian deputy foreign minister Faisal Mekdad told the BBC that armed groups supported by the United States and not the Syrian army had used chemical weapons in the conflict. He says US intervention would fuel terrorism and deepen hatred for America.
Mr Mekdad says intervention would also be a direct blow to the international community.
UN test results several weeks away
UN inspectors returned to the Netherlands at the weekend saying it could take up to three weeks to analyse the evidence they collected from the attack, although they were expected to give a preliminary report to UN Secretary-General Ban Ki-moon on Monday.
A spokesperson for the Secretary-General says the inspectors are working as fast as they can, within scientific constraints.
The United States says there is now firm evidence that sarin gas, a chemical nerve agent, was used by Syrian government forces in an attack last month that killed more than 1400 people, and President Barack Obama is seeking approval from Congress for a limited military strike against Syrian targets.
The BBC reports US Secretary of State John Kerry implied the evidence had been supplied by the United State's own sources, rather than via the UN inspectors. He says the samples provided tested positive for "signatures of sarin".
"We now have evidence from hair and blood samples from first responders in east Damascus, the people who came to help," he says.
Big push to persuade Congress
Mr Kerry's statement came a day after President Obama's announcement that he had formally asked Congress to authorise military action in Syria. The request will be considered at the next session of Congress beginning on 9 September.
Mr Kerry says he believes Congress will vote in favour of military action, although if Syria's President Bashar al-Assad is "foolish" enough to use chemical weapons again, Mr Obama would act quickly without necessarily seeking congressional approval.
The ABC's correspondent in Washington says Mr Obama and his top aides have launched a full-scale political offensive to persuade Congress to approve a limited military strike but most people believe that, if a vote were taken now, the answer would be no.
In advance of Congress's return from its summer recess next week, dozens of lawmakers cut short their holidays for an afternoon intelligence briefing on Syria with Mr Obama's national security team.
They reportedly raised a broad array of concerns, including the potential effectiveness of limited strikes, the possible unintended consequence of dragging the US into another open-ended Middle East conflict, the wisdom of acting without broader international backing to share the burden and the war fatigue of the American public.
Syria says the referral to Congress shows how complex the issue is and how paralysed Mr Obama is.
Deputy foreign minister Faisal Mekdad also says any US military action would be an act of aggression against Iran, which has been a steadfast ally of the Assad regime and supports its claim that it has not used chemical weapons.
However, an Iranian news agency is quoting a former president of Iran, Akhbar Hashimi Rafsanjani, as saying that the Syrian government has used chemical weapons against its own people.
The Arab League has also accused the Syrian government of carrying out last month's attack, but has so far stopped short of openly backing a military response. Foreign ministers of the league, now meeting in Cairo, have said only that the world should "take the deterrent and necessary measures against the culprits of this crime that the Syrian regime bears responsibility for".
Meanwhile the British-based Syrian Observatory for Human Rights, which relies on reports from activists, lawyers and doctors on the ground, says the death toll from the whole Syrian conflict over the past two-and-a-half years has now passed 110,000.