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.
Arab League foreign ministers called on the United Nations and the international community to take "deterrent" action against Syria.
The ministers, meeting in Cairo, said the world should "take the deterrent and necessary measures against the culprits of this crime that the Syrian regime bears responsibility for".
In a statement they also said those responsible should face war crimes trials.
UN experts have gathered evidence in Syria 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.
Syria has denied it was responsible and blames the rebels.
Mr 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 on Sunday.
"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.
Mr Kerry also said he was confident that Congress would give its approval for the US to launch strikes against Syria.
Congressmen "will do what is right because they understand the stakes", he said, declining to explain whether Mr Obama would press ahead even if Congress voted against.
Syrian Deputy Foreign Minister Faisal Mekdad told the BBC that Mr Obama's decision to delay the strikes pending a vote in Congress was just "a political and media manoeuvre".
Syrian President Bashar al-Assad remained defiant on Sunday, saying: "Syria... is capable of facing up to any external aggression just as it faces up to internal aggression every day, in the form of terrorist groups and those that support them."
The Syrian government has been fighting rebel forces since March 2011.
More than 100,000 people are estimated to have died in the conflict, and at least 1.7 million have become refugees.
'Failure in leadership'
Syria is known to have extensive supplies of chemical weapons.
Mr Obama has often said that using them would cross a "red line", prompting US intervention.
On Saturday, Mr 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.
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.
Forces which could be used against Syria:
•Four US destroyers - USS Gravely, USS Ramage, USS Barry and USS Mahan - are in the eastern Mediterranean, equipped with cruise missiles. The missiles can also be fired from submarines, but the US Navy does not reveal their locations
•Airbases at Incirlik and Izmir in Turkey, and in Jordan, could be used to carry out strikes
•Two aircraft carriers - USS Nimitz and USS Harry S Truman are in the wider region
•French aircraft carrier Charles de Gaulle is currently in Toulon in the western Mediterranean
•French Rafale and Mirage aircraft can also operate from Al-Dhahra airbase in the UAE