A year ago, Obama said that such an attack by the Syrian regime would cross a "red line," which he would not tolerate, but as he mulls military options, he is facing resistance even from those close to him.
Russia, which has a permanent seat on the U.N. Security Council, has said it would block any measure that includes military force against its ally, Syria.
Obama accused the council of being unable to "move in the face of a clear violation of international norms."
Kerry blamed this on "the guaranteed Russian obstructionism."
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Britain's Parliament has voted against joining any coalition.
But Kerry brushed off the vote, saying that the United States "makes our own decisions on our own time lines, based on our values and our interests."
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In Washington, some likened the claims that Syria had used chemical weapons on its own people to the claims -- made in the 2003 run-up to the U.S.-led war in Iraq -- that Baghdad had amassed weapons of mass destruction. They were never found.
Members of both parties in Congress have questioned the reliability of the U.S. intelligence.
More than 160 legislators, including 63 of Obama's fellow Democrats, signed letters calling for either a vote or at least a "full debate" before any U.S. action.
After meeting Friday with administration officials, Sen. Carl Levin, D-Michigan, said he had warned against "a kinetic strike" before the completion of the U.N. report and without the support of "a large number of nations, including Arab nations."
Levin, the chairman of the Senate Armed Services Committee, said he urged the administration to get lethal aid "to vetted elements of the Syrian opposition."
Senior administration officials are to discuss the August 21 attack in unclassified conference calls with the Senate Republican Conference and the Senate Democratic Caucus on Saturday afternoon, a White House official told CNN's Jill Dougherty.
Kerry has insisted that the Syrian situation differs from Iraq.
In this instance, he said, the intelligence community has "reviewed and re-reviewed" its information "more than mindful of the Iraq experience." He added: "We will not repeat that moment."
The president said he was not considering any option that would include "boots on the ground" or a long-term campaign.
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He previously ruled out setting up a no-fly zone.
Obama bemoaned international and domestic apprehensions. "A lot of people think something should be done, but nobody seems willing to do it."
"It's important for us to recognize that, when over 1,000 people are killed, including hundreds of innocent children, through the use of a weapon that 98 or 99% of humanity says should not be used even in war, and there is no action, then we're sending a signal that that international norm doesn't mean much," Obama said. "And that is a danger to our national security."
The president told reporters he had yet to make a final decision, but hinted at a military strike that sources and experts say would entail cruise missiles fired from U.S. Navy ships at Syrian command targets -- but not at any chemical weapons stockpiles.
Striking them could unleash poison gas that might kill more innocent civilians.
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Obama said he was looking at a "limited, narrow act" to ensure that Syria and others know the United States and its allies won't tolerate future violations.
But retired Maj. Gen. James Marks told CNN that the idea of a limited engagement could be an illusion: responses from Syria or others in the region could lead to extended entanglement.
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Former U.N. Chief Weapons Inspector Hans Blix, who failed to find weapons of mass destruction in Iraq before the U.S.-led invasion there, has called for the Security Council to condemn any use of chemical weapons anywhere in the world.
"It is a different thing to have a condemnation on behalf of the whole world by the world's highest council, the Security Council, rather than having it simply come out of Washington," he said.
The Russians could even take the initiative on the resolution, he said.
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"I think it's a secondary matter to point out who used it. Accountability can come later."
After the blanket condemnation, all parties supporting each side in the conflict should push them to a cease-fire, Blix said.
Military intervention would nix that possibility.
"Now we will have quarrels, if the U.S. goes ahead, rather than a united Security Council."
The Syrian government and the rebels may both have used the weapons, Blix said.
If the administration does attack, it may not have to do it alone.
Kerry cited support from the Arab League, Turkey and France.
French President Francois Hollande told Le Monde newspaper that intervention should be limited and not be directed toward overthrowing President Bashar al-Assad, a position shared by Obama.
But Turkey disagreed.
"The intervention shouldn't be a one- to two-day hit-and-run," Turkish Prime Minister Recep Tayyip Erdogan said late Friday. "It should bring the regime to the brink of giving up."
While the British vote was a blow to Obama's hopes of getting strong support from key NATO allies and some Arab League states, regional NATO ally Turkey on Friday backed the U.S. contention that al-Assad's regime was responsible for using chemical weapons.
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"The information at hand indicates that the opposition does not have these types of sophisticated weapons," said Foreign Minister Ahmet Davutoglu. "From our perspective, there is no doubt that the regime is responsible."
Australia also weighed in, with Prime Minister Kevin Rudd saying the evidence against al-Assad was overwhelming and, "therefore, the focus now legitimately lies on the most appropriate form of international response."
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CNN's Tom Cohen, Chelsea J. Carter, Barbara Starr, Lesa Jansen and Elise Labott contributed to this report.