U.S. officials have accused the Syrian government of using chemical weapons in an area outside Damascus recently, killing 1,429 people, including many children.
"It is the worst chemical attack in the 21st century," Obama said in an address on the White House lawn on Saturday. He also called the alleged chemical weapons attack "an assault on human dignity."
Obama said he would turn to Congress to get the green light to take military action, and that Congress is ready to debate and vote on the issue when it resumes Sept. 9.
Obama said the Syrian government's actions threaten allies of the U.S. that border Syria, such as Jordan and Israel, which could "lead to escalating use of chemical weapons or their proliferation to groups who want to do us harm."
"The U.S. should take military action against Syrian targets...This would not be open-ended or put boots on the ground but limited in design and scope."
With Navy ships in the Mediterranean Sea ready to strike, Obama said he had the power to unilaterally decide to attack but also determined "our country will be better off" if Congress renders its own opinion.
He then challenged lawmakers to consider "what message will we send to a dictator" if he is allowed to killed hundreds of children with chemical weapons without suffering any retaliation. Lawmakers will return to session on Sept. 9.
CBC Reporter Nahlah Ayed said there had been "great deal of anxiety" in the Middle East about a U.S. attack.
"Most people here were convinced a strike would happen," said Ayed from Beirut. "There is now relief that it's not imminent [and that] Obama's statement gives breathing space for everyone involved."
No deadline for strike
Obama emphasized the U.S. is prepared "to strike when we choose" and that could happen "tomorrow or next week or one month from now."
He went on to say his decision was not solely about Syria but about what other governments or groups could do.
"If we won't face this heinous act, what is our responsibility to other rules?" he said. "To governments that choose to build nuclear arms, to terrorists that would spread biological weapons, to armies that carry out genocide."
The president also admitted that talking about another U.S. military intervention may not be the most popular idea in a country that has been at war for more than a decade.
"I know that we are weary of war. We have ended one war in Iraq and ending another in Afghanistan," he said. "But we of the United States of America cannot turn a blind eye to Damascus.
"[It's] about who we are as a country ... that we do what we say and believe with the belief that right makes might, and not the other way around."
Putin urges caution
In Moscow, Russian President Vladimir Putin urged Obama not to rush into a decision.
The Russian leader said he was convinced the attack was a provocation carried out by those who want to draw the U.S. into the conflict, and that Washington should show any evidence to the contrary to the United Nations inspectors and the UN Security Council.
"If there is evidence it should be presented," Putin said. "If it is not presented, that means it does not exist."
Russia is one of Syrian President Bashar al-Assad's staunchest allies. Putin's comments were his first on the crisis since the suspected chemical weapons attack on rebel-held suburbs of Damascus on Aug. 21.
UN team ready to provide report
UN inspectors arrived in Amsterdam after spending several days in Syria collecting soil samples and interviewing victims of an attack last week in the Damascus suburbs. Officials said it could me more than a week before their final report is complete.
With the inspectors now out of Syria, the looming confrontation between the U.S. and Assad's regime moves one step closer to coming to a head. Most observers viewed U.S. military action as unlikely while the UN team was still inside Syria, but the Obama administration has made clear that it is confident in its assessment and could act before the UN releases the results of its probe.
U.S. officials accused the Assad regime of carrying out what it says was a chemical attack on Aug. 21 that killed at least 1,429 people — far more than previous estimates — including more than 400 children.
However, the Syrian Observatory for Human Rights, one of the main groups monitoring casualties in Syria, said Saturday it has only been able to confirm 502 deaths, identifying victims by name.
Syrian rebels prepare
Syrian rebels said they plan to go on the offensive against Assad’s troops in attacks timed to coincide with the U.S. strikes.
"As the (U.S.) strikes get under way, we will launch attacks against military airports, command centers, missile bases and military checkpoints," said Qassem Saadeddine, a senior rebel commander and spokesman for the rebels' Supreme Military Council.
Several other rebel commanders confirmed they would exploit the U.S. action to try and advance on the capital, the seat of Assad's power.
Still, some opposition fighters said they felt uneasy about the prospect of U.S. strikes on their country, calling the U.S. objectives vague and "suspicious."
Syrian state TV on Saturday morning broadcast footage of Syrian soldiers training, fighter jets soaring in the sky and tanks firing at unseen targets, all to the backdrop of martial music.