The US has evidence that the chemical nerve agent sarin was used in a deadly attack in Damascus last month, Secretary of State John Kerry says.
He said samples from hair and blood gathered after the attack "tested positive for signatures of sarin".
The US blames the Syrian government for the 21 August attack. President Barack Obama has vowed punitive action but wants Congress to vote on it first.
Syria dismissed the delay and said it was ready for any strike.
UN experts have been in Syria gathering evidence to determine whether chemical weapons attacks have taken place on various occasions. They have now arrived in the Netherlands with samples for analysis.
The biggest and deadliest apparent attack took place on 21 August in east Damascus. The US says more than 1,400 people were killed.
Damascus has returned to the usual pattern of the civil war. There's the loud crack of outgoing shellfire aimed at rebel-held suburbs, mixed with the Muslim call to prayer and Christian Sunday services.
Many Damascenes were expecting an imminent American attack, so there is a sense of relief, as if the tension has been broken. But the tension is sure to rise again, as the day of the congressional vote in Washington comes closer.
State television is still putting out strongly nationalist messages. It shows pictures of rockets firing, soldiers marching and tanks rampaging across the desert.
Washington said only the Damascus government has the capacity to launch such an attack.
Syria has denied it was responsible and blames the rebels.
Kerry implied that the US evidence was supplied by its own sources, rather than via the UN inspectors.
"In the last 24 hours, we have learned through samples that were provided to the United States that have now been tested from first responders in east Damascus and hair samples and blood samples have tested positive for signatures of Sarin," Kerry said on NBC's Meet The Press.
"So this case is building and this case will build."
The US has previously said it had similar evidence of sarin use in other attacks.
Kerry also said he was confident that Congress would give its approval for the US to launch strikes against Syria after it reconvenes on 9 September.
Syrian Deputy Foreign Minister Faisal Mekdad said Obama's decision to delay the strikes pending a vote in Congress was just "a political and media manoeuvre". He said Syria was "still prepared" for any attack.
The state-run Al-Thawra newspaper said Obama's decision was "the beginning of the historic American retreat". Syria is known to have extensive supplies of chemical weapons. Obama has often said that using them would cross a "red line", prompting US intervention.
On Saturday, Obama said any action would be limited, ruling out a ground invasion.
Congress is due to reconvene on 9 September, meaning any military operation would not happen until then.
The BBC's Jeremy Bowen in the Syrian capital, Damascus, says there is some relief in the city among those who feared that US attacks could start this weekend.
He says the delay could give the government time to move some sensitive military equipment.
BBC security correspondent Frank Gardner examines what is known about the 21 August attack
The opposition Syrian National Coalition (SNC) said President Obama's decision to delay any strikes in Syria was a "failure in leadership" and could "embolden" the forces of President Assad.
The UK has ruled out taking part in any attack, after Prime Minister David Cameron failed to win the support of parliament last week. That leaves France as the only other major power that has said it could strike against Syria - though it says it will not act on its own before the vote in the US Congress
Syrian President Bashar al-Assad has said his country will defend itself against any Western "aggression".