The United States says the chemical weapons used by Syrian government forces in Damascus last week killed 1429 people.
US Secretary of State John Kerry said on Friday the dead included 426 children.
In a live TV address from the White House, Mr Kerry revealed details of a US intelligence report that President Barack Obama is using to make the case for retaliation against the Syrian regime.
Mr Kerry said regime forces had prepared for the attack in Damascus three days earlier.
"We know rockets came only from regime-controlled areas and landed only in opposition-held areas," he said.
Mr Kerry said the administration will consult Congress and the American people over the next step.
Syria, which blames rebels for the event, said the US intelligence report is "entirely fabricated".
The foreign ministry has dismissed it as a poorly crafted document informed mostly by social media.
The BBC reports the UN Security Council is unlikely to approve any military intervention because permanent member Russia is a close ally of the Syrian government, and has vetoed two previous draft resolutions.
Russia says 'any unilateral military action bypassing the UN Security Council' would be a 'direct violation of international law.'
After Mr Kerry spoke, President Obama said the Security Council has shown an incapacity to act and part of America's obligation as a leader in the world is to make sure that when a government uses prohibited weapons, it is held to account.
He said a lot of people in the world think something needs to be done about the situation in Syria, but "nobody wants to do it".
The poison gas incident occurred on 21 August. Previous accounts put the number of dead at 335.
Mr Obama is using the report to make the case for retaliation against the Assad regime.
However, Mr Kerry stressed anything America might do would be carefully tailored.
He said it would not in any way resemble the US invasions of Afghanistan and Iraq, nor its intervention to help topple the former Libyan dictator Muammar Gaddafi.
Mr Kerry said the United States is not alone in its quest to punish the Syrian regime, citing the support of France and Australia.
Mr Obama said a limited, narrow military response is being planned.
Prime Minister Tayyip Erdogan of Turkey said on Friday any international military intervention against Syria should be aimed at bringing an end to the rule of President Assad.
He said the response cannot be a ''24-hour hit and run'' but must weaken the regime to the point where it gives up.
Mr Erdogan cited the NATO operation against Yugoslavia during the Kosovo war as an example.
Turkey says intelligence gathered by Ankara left no doubt that regime forces were responsible for the poison gas in Damascus.
French president determined to act
President Francois Hollande says a vote in the British parliament against military intervention in Syria won't affect France's will to act.
He still supports punitive action against the Syrian government because of the deaths in Damascus.
British MPs voted 285 to 272 on Thursday against involvement. Thirty Conservative and nine Liberal Democrat MPs voted against the motion.
Prime Minister David Cameron said it was clear Parliament did not want action and "the government will act accordingly".
Mr Hollande said the vote made no difference to France's position.
"Each country is sovereign to participate or not in an operation. That is valid for Britain as it is for France," he said.
He said that if the UN Security Council was unable to act, a coalition would form including the Arab League and European countries.
"But there are few countries which can have the capacity of enforcing any sanction through the appropriate measures," he said. "France will be part of it. France is ready."
Neither France nor the United States need parliamentary approval for action and Secretary of State John Kerry said the US could not be held to the foreign policy of others.
The French parliament is due to debate the issue before next Wednesday. The BBC reports Mr Hollande did not rule out the possibility that military action could be taken before then.
Meanwhile, Germany said on Friday it will not participate in any military action and Canada said no Canadian military mission is planned.
UN tests
President Bashar al-Assad denies that his forces used chemical weapons, blaming rebels.
UN chemical weapons inspectors visited a hospital in a government-controlled area of Damascus on Friday. The whole team left Syria on Saturday.
Secretary General Ban Ki-moon says the inspectors will brief him on their preliminary findings.
The BBC reports Mr Ban met the five permanent members of the Security Council in New York again on Friday.
They have held talks twice already this week, but after the last meeting on Thursday, diplomats said they were "far apart".
