But he said in Washington that such a gas attack represents a "challenge to the world."
"We cannot accept a world where women and children and innocent civilians are gassed on a terrible scale," he said.
He also said the Syrian chemical-weapons attack threatens U.S. allies such as Israel and Jordan.
Obama said that any military response would be a limited action and not an open-ended commitment.
"In no event are we considering any kind of military action that would involve boots on the ground. That would involve a long-term campaign," he said. "But we are looking at the possibility of a limited, narrow act that would help make sure that not only Syria but others around the world understand that the international community cares about maintaining this chemical-weapons ban and norm."
On August 30, the White House released unclassified details of evidence it says proves that Syrian regime forces ordered the August 21 attack on Damascus suburbs that killed more than 1,400 people, including women and children.
Among that evidence are details of intercepted phone calls between Syrian officials expressing concern that United Nations inspectors would connect the attack to the regime, and geospatial intelligence that shows government forces spent three days before the attack preparing to launch gas-filled rockets.
U.S. Secretary of State John Kerry summarized much of the evidence in remarks delivered in Washington earlier on August 30.
"We know where the rockets were launched from, and at what time. We know where they landed, and when. We know rockets came only from regime-controlled areas and went only to opposition-controlled or contested neighborhoods," he said.
He said the United States has seen "thousands of reports from 11 separate sites in the Damascus suburbs" showing people with symptoms of gas poisoning from the attacks.
He said the attacks killed a total of 1,429 people, including 426 children, and were a "crime against humanity." He said they demand an international response in order to assure that they don't happen again and that other states do not feel they can use chemical weapons with impunity.
"It matters here if nothing is done," Kerry said. "It matters if the world speaks out in condemnation and then nothing happens."
Kerry also said that an anticipated report from UN chemical-weapons inspectors who have visited some of the attack sites will only confirm that chemical weapons were used, but not by whom.
He said UN weapons inspectors "cannot tell the United States anything it does not already know" about the August 21 attacks and that Washington must make its own decisions based on its own timelines and values.
Finally, Kerry said that any punitive action which Washington launches against the Syrian regime will not be open-ended and that the United States will not assume responsibility for the Syrian civil war.
Kerry spoke as UN experts said they have finished their work in Syria and will now "expedite" a report on whether chemical weapons were used.
Meanwhile, U.S. Defense Secretary Chuck Hagel says the United States is still seeking an "international coalition" for possible military action against the Syrian regime, despite the British Parliament's vote against participating.
"Our approach is to continue to find an international coalition that will act together, and I think you're seeing a number of countries publicly state their position on the use of chemical weapons," he said. "We'll continue to consult with our allies and our partners and friends."
WATCH: U.S. activists protest possible military action
After the vote in London on August 29, the White House said U.S.-led strikes were still possible without the involvement of Britain, the United States' closest ally.
A statement said President Barack Obama's decisions "will be guided by what is in the best interests of the United States."
The Syrian government denies it used chemical weapons and has blamed rebels of carrying out the attack to trigger Western intervention.
British lawmakers opposed to military strikes had raised concerns that United Nations' investigators have not yet reported on their probe of the alleged attack, and that the UN Security Council has not voted in support of any outside military action.
Prime Minister David Cameron accepted the defeat for his government, saying he will not override the will of parliament.
"I strongly believe in the need for a tough response to the use of chemical weapons, but I also believe in respecting the will of this House of Commons," he said. "It is very clear tonight that while the House has not passed a motion, it is clear to me that the British Parliament, reflecting the views of the British people, does not want to see British military action -- I get that, and the government will act accordingly."
French President Francois Hollande said the British Parliament's decision would not affect his government's support for "proportional and firm action" against Assad's regime.
NATO chief Anders Fogh Rasmussen said the Western alliance would not be part of any strikes against Syria but also told reporters that the alleged use of chemical weapons demands “an international response, so it doesn't happen again."
A German government spokesman said "we haven't considered any German military participation and still aren't doing so."
A senior aide to Russian President Vladimir Putin said a military strike against Syria without UN approval would deal a "serious blow to the entire system of world order." Yuri Ushakov said the British vote showed "people were beginning to understand" the dangers of military action.
Russia, a strong Syrian ally, has blocked any measure at the UN Security Council that would authorize the use of force against Assad's regime.
On August 30, UN weapons inspectors visited a hospital in a government-held area of Damascus where soldiers affected by an apparent poison gas attack are reportedly being treated.
The UN inspectors plan to leave Syria by August 31.
Assad said on August 29 that Syria was ready to "defend itself against any aggression."
Israel Deploys Defense System
Meanwhile, in related news, Prime Minister Benjamin Netanyahu has announced that Israel has deployed its "Iron Dome" missile-defense system to guard against any attacks linked to the conflict in neighboring Syria.
Netanyahu said Israel is not involved in Syria’s civil war but is prepared to respond forcefully if it comes under attack. Israeli authorities have also ordered what’s described as a small-scale call-up of reserve soldiers, while queues have formed at some locations as ordinary Israelis seek to acquire gas masks.
Reports say that in an escalating conflict, U.S. ally Israel could face attacks from Syrian forces or the Shi’ite militant organization Hizballah, an ally of the Syrian regime.