Edging toward a punitive strike against Syria, President Barack Obama said he was weighing "limited and narrow" action as the Administration bluntly accused Bashar Assad's Government of launching a chemical weapons attack that killed at least 1429 people, far more than previous estimates, including more than 400 children.
No "boots on the ground", Obama said, seeking to reassure Americans weary after a decade of war in Iraq and Afghanistan.
With France as his only major public ally, Obama said he strongly preferred multilateral action.
"Frankly, part of the challenge we end up with here is a lot of people think something should be done but nobody wants to do it."
United States warships in place in the Mediterranean Sea carry cruise missiles that can find targets hundreds of kilometres away without the need of air cover or troops on the ground.
In what appeared increasingly like the pre-attack endgame, UN personnel dispatched to Syria carried out a fourth and final day of inspection as they sought to determine precisely what happened in last week's attack.
The international contingent left Syria early yesterday and crossed into Lebanon. They were heading to laboratories in Europe with the samples they have collected.
Video said to be taken at the scene shows victims writhing in pain, twitching and exhibiting symptoms associated with exposure to nerve agents. The videos distributed by activists to support their claims of a chemical attack were consistent with reports of shelling in the suburbs of Damascus at the time, though it was not known if the victims had died from a poisonous gas attack.
The Syrian Government said the US claims were "flagrant lies", akin to Bush Administration assertions before the Iraq invasion that Saddam Hussein had weapons of mass destruction.
A Foreign Ministry statement read on state television said that "under the pretext of protecting the Syrian people, they are making a case for an aggression that will kill hundreds of innocent Syrian civilians".
Residents of Damascus have stocked up on food and other necessities in anticipation of strikes. One man, 42-year-old Talal Dowayih, said: "I am not afraid from the Western threats to Syria; they created the chemical issue as a pretext for intervention, and they are trying to hit Syria for the sake of Israel."
At the White House, Obama met national security aides and diplomats from Baltic countries, saying he had not yet decided on a response to the attack.
But the Administration did nothing to discourage predictions that he would act, and soon. Strongly worded remarks from Secretary of State John Kerry heightened the impression, along with the release of an unclassified assessment that cited "high confidence" that the Syrian Government carried out the attack.
The assessment reported that about 3600 patients "displaying symptoms consistent with nerve agent exposure" were seen at Damascus hospitals after the attack. To that, Kerry added "a senior regime official who knew about the attack confirmed that chemical weapons were used by the regime".
The assessment did not explain the unexpectedly large casualty count, far in excess of an estimate from Doctors Without Borders. It also did not say expressly how the US knew what one Syrian official had allegedly said to another.
Mindful of public opinion, Kerry urged Americans to read the four-page assessment. He referred to Iraq, when Bush Administration assurances that weapons of mass destruction were present proved false, and a US invasion led to a long, deadly war. Kerry said this time it would be different.
The nation's top diplomat said: "It is directly related to our credibility and whether countries still believe the US when it says something. They are watching to see if Syria can get away with it because then maybe they, too, can put the world at greater risk."
While Obama was having trouble enlisting foreign support, French President Francois Hollande issued a statement saying the two leaders had "agreed that the international community cannot tolerate the use of chemical weapons, that it must hold the Syrian regime responsible and send a strong message to denounce the use of [such] arms".
The looming confrontation is the latest outgrowth of a civil war in which Assad has brutally clung to power. An estimated 100,000 civilians have been killed in more than two years, many of them from Government attacks.
Obama declared more than a year ago that the use of chemical weapons would amount to a "red line" that Assad should not cross.
With memories of the long Iraq war still fresh, the political crosscurrents have been intense both domestically and overseas.
Dozens of politicians, most of them Republican, have signed a letter saying Obama should not take military action without congressional approval. Top leaders of both political parties are urging the President to consult more closely with Congress before giving an order to launch hostilities.
UN Secretary-General Ban Ki-moon has urged a delay in any military action until the inspectors can present their findings to UN member states and the Security Council.