While he said he had made no "final decision" on taking military action against Bashar al-Assad's regime, Obama gave his clearest indication yet that an attack is imminent.
His remarks came after the United States released an intelligence report that concluded the regime had launched a chemical onslaught in the suburbs of Damascus last week, killing 1,429 people, including at least 426 children.
"This kind of attack is a challenge to the world," Obama said, in brief remarks to reporters at the White House.
"We cannot accept a world where women and children and innocent civilians are gassed on a terrible scale," he said, calling the attack a threat to US "national security interests."
"I have said before, and I meant what I said, that the world has an obligation to make sure we maintain the norm against the use of chemical weapons," he said, slamming the "incapacity" of the UN Security Council to act.
Obama said his administration and the military were looking at a "wide range of options" but had ruled out "boots on the ground" or a "long-term campaign."
"We are looking at the possibility of a limited, narrow act," he said.
France gave its backing to the US plans, but British lawmakers voted against any involvement in military action and other close US allies said they would not sign up.
Russia, Syria's most powerful ally, has meanwhile questioned US intelligence on the August 21 gas attacks and has warned against any military strikes without UN backing.
In Damascus, UN experts completed their investigation into the attacks east of the capital and said they would "expedite" a report on whether chemical weapons had been used there.
The team is due to leave the war-battered country Saturday and report back immediately to UN chief Ban Ki-moon, who has appealed to the West to allow time for their findings to be assessed.
US Secretary of State John Kerry cited "multiple streams of intelligence" indicating that the Syrian government had carried out the attack and that Assad himself is the "ultimate decision maker" for the country's chemical weapons program.
Kerry said failure to act would not only erode the nearly century-old norm against the use of chemical weapons, but would embolden Syrian allies Iran and Hezbollah.
But the United States, faced with an impasse at the Security Council and the British parliament's shock vote Thursday, has been forced to look elsewhere for international partners.
While Germany and Canada ruled out joining any military strikes, French President Francois Hollande -- whose country was a strident opponent of the US-led war on Iraq -- said the British decision would not affect his government's stance.
"France wants firm and proportionate action against the Damascus regime," Hollande said in an interview with Le Monde newspaper.
Turkey, Syria's neighbor, went further still, demanding not just surgical strikes to send a message about chemical weapons but a sustained campaign to topple the regime.
"A limited operation cannot be satisfactory for us," Recep Tayyip Erdogan was quoted as saying by the NTV news channel.
"An intervention akin to that decided for Kosovo must be launched. An operation of one or two days will not be enough. The goal should be to force the regime out."
US Defense Secretary Chuck Hagel said the White House was still seeking an "international coalition" against Assad's regime.
The British government's defeat in parliament came after the failure of an 11th-hour effort by British diplomats to win UN backing for action at a meeting of Security Council permanent members.
Gruesome pictures of some of the reported victims of the attacks, including children, have shocked the world and piled on the pressure for a response that could draw a reluctant West into the vicious Syrian civil war.
But Russia and Iran, and even some US allies, have warned against any intervention, saying it risked sparking a wider conflict.
Divisions over Syria have further chilled the frosty relations between Washington and Moscow ahead of the G20 summit next week in Saint Petersburg, where pointedly there will be no face-to-face talks between Obama and Russian President Vladimir Putin.
Earlier in the week, reports had suggested a Western strike was imminent, but questions had been raised about the quality of the intelligence linking Assad to the gas attack.
Syria's Foreign Minister Walid Muallem reportedly told the UN chief that Damascus would reject any "partial" conclusions about the attack before full analyses are undertaken.
The military buildup was continuing in the region, with US warships armed with scores of cruise missiles converging on the eastern Mediterranean.
In Damascus the mood was heavy with fear and security forces were making preparations for possible air bombardments, pulling soldiers back from potential targets.
More than 100,000 people have died since the conflict erupted in March 2011 and two million have become refugees, half of them children, according to the United Nations.
Some commentators have questioned the wisdom of Obama dragging the United States into another conflict after the wars in Iraq and Afghanistan -- particularly as Al-Qaeda militants are among the rebels fighting the Assad regime.
Some members of the US Congress voiced support for limited surgical missile strikes, while urging transparency from the administration.
But an opinion poll released Friday said half of all Americans believe Obama should not intervene, and Italian Foreign Minister Emma Bonino said military strikes risked escalating the conflict into a "global conflagration".
Putin's chief foreign policy aide Uri Ushakov said any military action that bypasses the Security Council "will deal a serious blow to the entire system of world order."
Assad, whose regime denies using chemical weapons and instead blames "terrorist" rebels for last week's attack, remains defiant.
"Syria will defend itself in the face of any aggression," he said Thursday, vowing "victory" for his people.
COMMENTS (7)
Comments are moderated and generally will be posted if they are on-topic and not abusive.
For more information, please see our Comments FAQ
"Obama says World cannot stand by on Syria, the correct headline/statement above should have been as apparent from the picture of "Obama says world cannot stand by for USA" every time.
The world couldn't stand by on WMDs in Eye-rack either, but where are they? Anyone?
Shouldn't Israel be investigated along with the "usual Arab crooks". Its necessary at least on paper.
@JD, He will win it again for saving innocent children from the chemical weapons. But the world wants more of it.
Yes, Mr. Obama, Good job. Go ahead.
Wait. Obama's the same guy who got a nobel prize for peace or something... right?
If US mischief was not so ineradicably tragic it would be amusing. The Syrian beat up is just a tired old replay of America's illegal attack on Iraq. Fortunately, it does not appear that Pakistan is getting involved.