The United States has made it clear that it would punish the Syrian President, Bashar al-Assad, for the "brutal and flagrant" chemical weapons attack that it said killed 1,429 people near Damascus last week.
The US Secretary of State, John Kerry, said on Friday that the dead included 426 children.
In a speech at the State Department, he described the attack as an "inconceivable horror".
The US government released a four-page document on Friday outlining their conclusions about the August 21 attack, which include:
- 1,429 people were killed, including "at least 426 children";
- Beginning on August 18, the army was involved in "preparations for a chemical weapons attack";
- "A senior official intimately familiar with the offensive confirmed that chemical weapons were used by the regime"
President Barack Obama later said the US was considering a "limited narrow act" in response to the attack, which posed "a challenge to the world".
"We are not considering any kind of military action that involves boots on the ground or a long-term campaign," he said.
But Obama said the use of chemical weapons threatens US national security and merits a response.
The Syrian Foreign Ministry has dismissed Kerry's statement as "full of lies" based on "fabricated evidence", insisting the rebels carried out the deadly attack.
State-run news agency SANA said Kerry, who cited a US intelligence report, was using "material based on old stories that the terrorists have been circulating for more than a week".
The intelligence gathered for the US report included an intercepted communication by a senior official intimately familiar with the August 21 attack as well as other intelligence from people's accounts and intercepted messages, the four-page report said.
A team of UN experts finished gathering evidence of the alleged poison-gas attacks, the United Nations announced on Friday, though envoys said analysing the samples may take weeks.
Kerry said their report would only confirm that chemical weapons were used, and he made clear that would not change much for Washington since "guaranteed Russian obstructionism" would make it impossible for the UN to galvanise world action.
"The primary question is really no longer, what do we know. The question is, what are we - we collectively - what are we in the world going to do about it," Kerry said.
He said the president had been clear that any action would be "limited and tailored" to punishing Assad, that it would not be intended to affect the civil war there and Washington remained committed to a diplomatic solution to the crisis.
Al Jazeera's Patty Culhane, reporting from Washington, said: "What the administration is really considering is some kind of targeted cruise-missile strike, that would be directed at command-and-control units of the Syrian military, fuel depots, possibly some runways. But as the president just said, it should stay limited."
Kerry and Obama were speaking the day after British Prime Minister David Cameron failed to win parliamentary backing for military action in Syria.
Kerry made clear Washington would not be swayed from acting either by the opinions of other states: "President Obama will ensure that the United States of America makes our own decisions on our own timelines, based on our values and our interests."
He said that if a "thug and a murderer like Bashar al-Assad can gas thousands of his own people with impunity," it would be an example to others, such as, he said, Iran, Hezbollah and North Korea.
"Will they remember that the Assad regime was stopped from those weapons' current or future use? Or will they remember that the world stood aside and created impunity?" Kerry said.
French President Francois Hollande told the daily Le Monde he still supported taking "firm" punitive action over an attack he said had caused "irreparable" harm to the Syrian people.
Hollande is not constrained by the need for parliamentary approval of any move to intervene in Syria and could act before lawmakers debate the issue on Wednesday.
An official in his office said the French president spoke to Obama on Friday and "they showed great determination about this crisis".
"Both heads of state agreed that the international community cannot tolerate the use of chemical weapons, that it should hold the Syrian regime accountable for it and send a strong message," a statement from the Elysee palace said.
"As close allies and friends, France and the United States will continue their consultation on Syria and on all other issues affecting international security," the statement said.
The White House issued an identical statement after the phone call, which an aide to Hollande said lasted around 45 minutes.