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".
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.
However, the UN Security Council is unlikely to approve any military intervention because permanent member Russia is a close ally of the Syrian government.
Russia, along with China, has vetoed two previous draft resolutions on Syria.
The US was also dealt a blow on Thursday when the UK parliament rejected a motion supporting the principle of military intervention.
The vote rules the UK out of any potential military alliance.
British Prime Minister David Cameron and Mr Obama spoke over the telephone on Friday, agreeing to continue to co-operate on international issues.
The president told Mr Cameron he "fully respected" the approach taken by the UK government, according to the prime minister's office.
US officials said they would continue to push for a coalition, and France said it was ready to take action in Syria alongside the US.
Neither France nor the US need parliamentary approval for action.
French President Francois Hollande, who also spoke to Mr Obama late on Friday, said the two men had agreed that the international community must "send a strong message" denouncing chemical attacks.
Another US ally, Turkey, called for action similar to the Nato bombing raids in the former Yugoslavia in 1999.
Nato carried out 70 days of air strikes to protect civilians from attack in Kosovo, despite not having a UN resolution.
The use of chemical weapons is banned under several treaties, and considered illegal under customary international humanitarian law.
The Syrian army is known to have stockpiles of chemical agents including sarin gas.
Earlier accounts of the attack in Damascus quoted officials from medical charity Medicins Sans Frontieres as saying 355 people had been killed.
The UN inspectors have collected various samples that will now be examined in laboratories across the world.
The UN team is not mandated to apportion blame for the attacks.
More than 100,000 people are estimated to have died since the conflict erupted in Syria in March 2011, and the conflict has produced at least 1.7 million refugees.
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 Raffale and Mirage aircraft can also operate from Al-Dhahra airbase in the UAE